Add -s Slurp Flag

[yhakbar]

Describe the solution you'd like
Reversibility of the boilerplate command would be beneficial for iteration on templates, as it would allow for updates to a templated directory to be slurped back up into the template doing the substitutions in reverse (value to placeholder).

Adding something like a .boilerplate-manifest file that has a list of files that have been stamped out and the substitutions performed in them might help to make reversibility simpler (and potentially faster).

Describe alternatives you've considered
The alternative is to manually update the template being iterated on while updating the templated directory.