added new `GPM::getInstallable()` method
added new `GPM::getInstallable()` method

Vivalldi/grav,sylvestrelucia/grav,joeyhenricks/a-broke-vegan,userfrosting/learn,Sommerregen/grav,JamesPan/gravbox,shortsofia/testesitegrav.github.io,tomzx/blog.tomrochette.com,sylvestrelucia/grav,sylvestrelucia/grav,userfrosting/learn,notklaatu/grav,tomzx/blog.tomrochette.com,JamesPan/gravbox,webuxr/iamrs,dimayakovlev/grav,tomzx/blog.tomrochette.com,getgrav/grav,stell/grav,ccardinaux/grav-test,tomzx/blog.tomrochette.com,sylvestrelucia/grav,JamesPan/gravbox,dimayakovlev/grav,gigago/grav,Vivalldi/grav,JamesPan/gravbox,userfrosting/learn,joeyhenricks/a-broke-vegan,getgrav/grav,userfrosting/learn,notklaatu/grav