ENH: Teach pre-commit to validate migration guide XML
ENH: Teach pre-commit to validate migration guide XML

Use xmllint if it is available.  Otherwise instruct the user how to
manually validate the xml files.

Change-Id: I6f879a249c57b10c3f41f312371551ab67a046c0

vfonov/ITK,richardbeare/ITK,rhgong/itk-with-dom,stnava/ITK,stnava/ITK,msmolens/ITK,biotrump/ITK,heimdali/ITK,paulnovo/ITK,BRAINSia/ITK,blowekamp/ITK,fbudin69500/ITK,hinerm/ITK,ajjl/ITK,atsnyder/ITK,vfonov/ITK,rhgong/itk-with-dom,BRAINSia/ITK,hjmjohnson/ITK,jcfr/ITK,LucHermitte/ITK,jcfr/ITK,malaterre/ITK,malaterre/ITK,GEHC-Surgery/ITK,hjmjohnson/ITK,blowekamp/ITK,rhgong/itk-with-dom,Kitware/ITK,richardbeare/ITK,fedral/ITK,jcfr/ITK,thewtex/ITK,biotrump/ITK,fbudin69500/ITK,PlutoniumHeart/ITK,hendradarwin/ITK,GEHC-Surgery/ITK,paulnovo/ITK,fuentesdt/InsightToolkit-dev,fuentesdt/InsightToolkit-dev,eile/ITK,BRAINSia/ITK,blowekamp/ITK,fbudin69500/ITK,LucHermitte/ITK,paulnovo/ITK,fuentesdt/InsightToolkit-dev,GEHC-Surgery/ITK,GEHC-Surgery/ITK,zachary-williamson/ITK,ajjl/ITK,fuentesdt/InsightToolkit-dev,heimdali/ITK,CapeDrew/DCMTK-ITK,blowekamp/ITK,paulnovo/ITK,eile/ITK,blowekamp/ITK,jmerkow/ITK,BRAINSia/ITK,hinerm/ITK,thewtex/ITK,thewtex/ITK,heimdali/ITK,fuentesdt/InsightToolkit-dev,spinicist/ITK,fedral/ITK,atsnyder/ITK,hjmjohnson/ITK,eile/ITK,biotrump/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,LucasGandel/ITK,hjmjohnson/ITK,eile/ITK,hendradarwin/ITK,hinerm/ITK,ajjl/ITK,jcfr/ITK,stnava/ITK,rhgong/itk-with-dom,malaterre/ITK,Kitware/ITK,fuentesdt/InsightToolkit-dev,PlutoniumHeart/ITK,vfonov/ITK,GEHC-Surgery/ITK,InsightSoftwareConsortium/ITK,fuentesdt/InsightToolkit-dev,vfonov/ITK,LucHermitte/ITK,hendradarwin/ITK,msmolens/ITK,Kitware/ITK,PlutoniumHeart/ITK,zachary-williamson/ITK,CapeDrew/DCMTK-ITK,Kitware/ITK,richardbeare/ITK,msmolens/ITK,msmolens/ITK,paulnovo/ITK,malaterre/ITK,LucasGandel/ITK,spinicist/ITK,CapeDrew/DCMTK-ITK,rhgong/itk-with-dom,msmolens/ITK,Kitware/ITK,fedral/ITK,hjmjohnson/ITK,ajjl/ITK,BRAINSia/ITK,jcfr/ITK,jmerkow/ITK,ajjl/ITK,stnava/ITK,jcfr/ITK,PlutoniumHeart/ITK,fedral/ITK,biotrump/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,fuentesdt/InsightToolkit-dev,fedral/ITK,LucasGandel/ITK,BlueBrain/ITK,thewtex/ITK,PlutoniumHeart/ITK,heimdali/ITK,atsnyder/ITK,fbudin69500/ITK,fedral/ITK,InsightSoftwareConsortium/ITK,jmerkow/ITK,eile/ITK,BlueBrain/ITK,jmerkow/ITK,rhgong/itk-with-dom,jmerkow/ITK,spinicist/ITK,paulnovo/ITK,eile/ITK,stnava/ITK,rhgong/itk-with-dom,spinicist/ITK,fbudin69500/ITK,paulnovo/ITK,vfonov/ITK,CapeDrew/DCMTK-ITK,stnava/ITK,BRAINSia/ITK,thewtex/ITK,spinicist/ITK,biotrump/ITK,rhgong/itk-with-dom,hendradarwin/ITK,LucHermitte/ITK,hendradarwin/ITK,atsnyder/ITK,fbudin69500/ITK,zachary-williamson/ITK,atsnyder/ITK,heimdali/ITK,malaterre/ITK,GEHC-Surgery/ITK,hinerm/ITK,vfonov/ITK,zachary-williamson/ITK,malaterre/ITK,jmerkow/ITK,malaterre/ITK,atsnyder/ITK,BlueBrain/ITK,CapeDrew/DCMTK-ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,malaterre/ITK,spinicist/ITK,heimdali/ITK,heimdali/ITK,CapeDrew/DCMTK-ITK,jmerkow/ITK,hjmjohnson/ITK,hendradarwin/ITK,hendradarwin/ITK,spinicist/ITK,vfonov/ITK,ajjl/ITK,ajjl/ITK,fuentesdt/InsightToolkit-dev,stnava/ITK,biotrump/ITK,fedral/ITK,spinicist/ITK,richardbeare/ITK,GEHC-Surgery/ITK,stnava/ITK,blowekamp/ITK,zachary-williamson/ITK,hinerm/ITK,Kitware/ITK,richardbeare/ITK,LucasGandel/ITK,spinicist/ITK,fedral/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,hinerm/ITK,zachary-williamson/ITK,hinerm/ITK,zachary-williamson/ITK,ajjl/ITK,LucasGandel/ITK,BlueBrain/ITK,thewtex/ITK,thewtex/ITK,biotrump/ITK,hinerm/ITK,hjmjohnson/ITK,atsnyder/ITK,CapeDrew/DCMTK-ITK,msmolens/ITK,jcfr/ITK,malaterre/ITK,heimdali/ITK,jmerkow/ITK,LucHermitte/ITK,BlueBrain/ITK,BlueBrain/ITK,msmolens/ITK,paulnovo/ITK,eile/ITK,zachary-williamson/ITK,fbudin69500/ITK,GEHC-Surgery/ITK,atsnyder/ITK,hendradarwin/ITK,fbudin69500/ITK,hinerm/ITK,blowekamp/ITK,LucasGandel/ITK,LucHermitte/ITK,atsnyder/ITK,eile/ITK,msmolens/ITK,biotrump/ITK,vfonov/ITK,LucHermitte/ITK,richardbeare/ITK,BlueBrain/ITK,eile/ITK,CapeDrew/DCMTK-ITK,BlueBrain/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,jcfr/ITK,LucHermitte/ITK,PlutoniumHeart/ITK,CapeDrew/DCMTK-ITK,LucasGandel/ITK,richardbeare/ITK,blowekamp/ITK,stnava/ITK,PlutoniumHeart/ITK