add get_item convenience function to fetch Items by id
add get_item convenience function to fetch Items by id

The beetfs project uses this to quickly get paths for items when reading them.

madmouser1/beets,pdf/beets,shamangeorge/beets,xsteadfastx/beets,imsparsh/beets,drm00/beets,swt30/beets,jbaiter/beets,LordSputnik/beets,jackwilsdon/beets,artemutin/beets,xsteadfastx/beets,Freso/beets,artemutin/beets,Andypsamp/CODfinalJUNIT,randybias/beets,tima/beets,marcuskrahl/beets,SusannaMaria/beets,MyTunesFreeMusic/privacy-policy,xsteadfastx/beets,Kraymer/beets,kareemallen/beets,marcuskrahl/beets,ttsda/beets,mried/beets,Andypsamp/CODfinalJUNIT,madmouser1/beets,lightwang1/beets,kareemallen/beets,PierreRust/beets,sadatay/beets,sadatay/beets,xsteadfastx/beets,mathstuf/beets,parapente/beets,Wen777/beets,dfc/beets,multikatt/beets,LordSputnik/beets,artemutin/beets,sampsyo/beets,mosesfistos1/beetbox,ruippeixotog/beets,ibmibmibm/beets,randybias/beets,m-urban/beets,madmouser1/beets,PierreRust/beets,andremiller/beets,mried/beets,untitaker/beets,Dishwishy/beets,kelvinhammond/beets,LordSputnik/beets,mried/beets,asteven/beets,imsparsh/beets,lightwang1/beets,m-urban/beets,sadatay/beets,randybias/beets,drm00/beets,ttsda/beets,Freso/beets,Andypsamp/CODfinalJUNIT,lengtche/beets,ttsda/beets,m-urban/beets,sadatay/beets,jayme-github/beets,marcuskrahl/beets,bj-yinyan/beets,sampsyo/beets,swt30/beets,jackwilsdon/beets,Freso/beets,SusannaMaria/beets,imsparsh/beets,mathstuf/beets,jcoady9/beets,parapente/beets,jmwatte/beets,swt30/beets,diego-plan9/beets,lengtche/beets,Freso/beets,kelvinhammond/beets,tima/beets,Dishwishy/beets,aspidites/beets,lengtche/beets,kelvinhammond/beets,kareemallen/beets,beetbox/beets,Dishwishy/beets,mosesfistos1/beetbox,gabrielaraujof/beets,bj-yinyan/beets,asteven/beets,untitaker/beets,ruippeixotog/beets,Wen777/beets,diego-plan9/beets,bj-yinyan/beets,pkess/beets,drm00/beets,moodboom/beets,untitaker/beets,kelvinhammond/beets,ngokevin/beets,jmwatte/beets,Kraymer/beets,Andypsamp/CODjunit,parapente/beets,ibmibmibm/beets,swt30/beets,madmouser1/beets,tima/beets,bj-yinyan/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,mathstuf/beets,jayme-github/beets,PierreRust/beets,mathstuf/beets,moodboom/beets,ngokevin/beets,arabenjamin/beets,Dishwishy/beets,lightwang1/beets,dfc/beets,mclearc/beets,Kraymer/beets,aspidites/beets,pkess/beets,MyTunesFreeMusic/privacy-policy,gabrielaraujof/beets,lightwang1/beets,pkess/beets,arabenjamin/beets,SusannaMaria/beets,PierreRust/beets,Wen777/beets,Kraymer/beets,andremiller/beets,dfc/beets,drm00/beets,jackwilsdon/beets,YetAnotherNerd/beets,mclearc/beets,krig/beets,asteven/beets,YetAnotherNerd/beets,multikatt/beets,Andypsamp/CODjunit,marcuskrahl/beets,m-urban/beets,shanemikel/beets,diego-plan9/beets,MyTunesFreeMusic/privacy-policy,moodboom/beets,Andypsamp/CODfinalJUNIT,tima/beets,Andypsamp/CODjunit,beetbox/beets,ruippeixotog/beets,pdf/beets,YetAnotherNerd/beets,shanemikel/beets,shamangeorge/beets,diego-plan9/beets,ttsda/beets,andremiller/beets,shamangeorge/beets,dfc/beets,jackwilsdon/beets,mosesfistos1/beetbox,parapente/beets,beetbox/beets,jmwatte/beets,jbaiter/beets,krig/beets,YetAnotherNerd/beets,SusannaMaria/beets,jcoady9/beets,untitaker/beets,gabrielaraujof/beets,ruippeixotog/beets,mclearc/beets,Andypsamp/CODjunit,mosesfistos1/beetbox,multikatt/beets,Andypsamp/CODjunit,Andypsamp/CODfinalJUNIT,jmwatte/beets,randybias/beets,mried/beets,moodboom/beets,imsparsh/beets,lengtche/beets,arabenjamin/beets,kareemallen/beets,jcoady9/beets,shanemikel/beets,multikatt/beets,LordSputnik/beets,sampsyo/beets,beetbox/beets,asteven/beets,pkess/beets,gabrielaraujof/beets,shanemikel/beets,jcoady9/beets,ibmibmibm/beets,artemutin/beets,arabenjamin/beets,shamangeorge/beets,krig/beets,sampsyo/beets