chroot: bind path and insert package options.

[bartoszek]

I would like to take a swing at paur but there's a few missing features to facilitate my current build routine, specifically the chroot subsystem lacks:

• flag and config section to bind arbitrary path to chroot: required for ccache (currently only local repos gets binded)
• flag and config section to insert extra packages into chroot: required for distcc, ccache, ccache-ext, makepkg-optimize