Fix crash when markdown_extensions is not set
Fix crash when markdown_extensions is not set

This lets the value be a tuple as well as a list. (Never really understood why
Python won't concatenate lists and tuples.)

d0ugal/mkdocs,gregelin/mkdocs,simonfork/mkdocs,simonfork/mkdocs,cazzerson/mkdocs,simonfork/mkdocs,kubikusrubikus/mkdocs,jamesbeebop/mkdocs,jpush/mkdocs,peter1000/mkdocs,waylan/mkdocs,vi4m/mkdocs,xeechou/mkblogs,mkdocs/mkdocs,samuelcolvin/mkdocs,d0ugal/mkdocs,jpush/mkdocs,wenqiuhua/mkdocs,vi4m/mkdocs,justinkinney/mkdocs,ramramps/mkdocs,xeechou/mkblogs,xeechou/mkblogs,nicoddemus/mkdocs,jimporter/mkdocs,pjbull/mkdocs,samhatfield/mkdocs,dmehra/mkdocs,dmehra/mkdocs,pjbull/mkdocs,samuelcolvin/mkdocs,peter1000/mkdocs,lukfor/mkdocs,gregelin/mkdocs,dmehra/mkdocs,justinkinney/mkdocs,tedmiston/mkdocs,d0ugal/mkdocs,vi4m/mkdocs,rickpeters/mkdocs,kubikusrubikus/mkdocs,mlzummo/mkdocs,jimporter/mkdocs,michaelmcandrew/mkdocs,lukfor/mkdocs,ericholscher/mkdocs,waylan/mkdocs,jpush/mkdocs,wenqiuhua/mkdocs,wenqiuhua/mkdocs,jpush/mkdocs,hhg2288/mkdocs,davidgillies/mkdocs,ericholscher/mkdocs,mlzummo/mkdocs,longjl/mkdocs,justinkinney/mkdocs,longjl/mkdocs,jamesbeebop/mkdocs,williamjmorenor/mkdocs,hhg2288/mkdocs,jimporter/mkdocs,cazzerson/mkdocs,wenqiuhua/mkdocs,ramramps/mkdocs,cnbin/mkdocs,kubikusrubikus/mkdocs,mlzummo/mkdocs,samhatfield/mkdocs,fujita-shintaro/mkdocs,davidgillies/mkdocs,fujita-shintaro/mkdocs,tedmiston/mkdocs,ramramps/mkdocs,jeoygin/mkdocs,lbenet/mkdocs,lukfor/mkdocs,samuelcolvin/mkdocs,samhatfield/mkdocs,tedmiston/mkdocs,lbenet/mkdocs,xeechou/mkblogs,williamjmorenor/mkdocs,cnbin/mkdocs,cazzerson/mkdocs,mkdocs/mkdocs,fujita-shintaro/mkdocs,cazzerson/mkdocs,gregelin/mkdocs,jeoygin/mkdocs,jamesbeebop/mkdocs,nicoddemus/mkdocs,jeoygin/mkdocs,mkdocs/mkdocs,nicoddemus/mkdocs,michaelmcandrew/mkdocs,peter1000/mkdocs,rickpeters/mkdocs,waylan/mkdocs,longjl/mkdocs,cnbin/mkdocs,ericholscher/mkdocs,hhg2288/mkdocs,rickpeters/mkdocs,davidgillies/mkdocs,lbenet/mkdocs,michaelmcandrew/mkdocs,pjbull/mkdocs,williamjmorenor/mkdocs