Add ability to run a command before deploying

[asmeurer]

For the SymPy docs we run a command that post-processes the docs to add links to the other versions. Doctr needs the ability to run a command after the docs are moved in place but before they are committed.

[asmeurer]

This isn't so straightforward with the way commit_docs currently works (and by the way, commit_docs doesn't actually commit, it just adds files to the index). So some internal refactoring may be in order.