Support removal/rename of files upon update/install

[vgmoose]

This issue is for creating a new manifest option for just deleting a file at a given path.

eg.

D: sept/payload.bin

Depending on need and feasibility, a rename option might also be a good idea, eg:

M: sept/payload.bin : old/sept/payload.bin

being equivalent to mv sept/payload.bin old/sept/payload.bin

If there are any other ideas for some more metadata options, they can go in this ticket as well

[vgmoose]

I should also say that this change should go through on the manifest branch, where all the logic for updating/removing from a libget manifest is to be encapsulated

[vgmoose]

As of #8 if a currently installed package has a manifest with a file that’s U or E, and then that file IS NOT present in an update’s manifest, it will be deleted upon upgrade

This automatically can address a case this issue tries to cover, but there is still a use case for a pure delete outside of package scope