Add magit-todos-filename-modifier customization option

[matsievskiysv]

Apply function stored in magit-todos-filename-modifier to the filenames. Closes #105

For example, this is the default view with full path:

And with function f-filename applied:

[alphapapa]

That definitely helps in that situation.

Regarding the implementation, I think it would probably be better to do something like:

(when magit-todos-trim-filename-fn
  (setf filename (funcall magit-todos-trim-filename-fn)))

This avoids making a function call when the option is disabled. As well, funcall can be used instead of apply, which avoids having to cons another list.

Thanks.

[matsievskiysv]

all done

[alphapapa]

Thanks for your patience on this. If you're still interested, I have a few more comments (apologies if any go in circles; it's been a while, so my tastes may have changed since the last time I reviewed this, as I've learned more about Elisp)...

[matsievskiysv]

All done

[alphapapa]

@matsievskiysv Thanks for your patience. I couldn't update the PR because you made it from your master branch, but I rebased and merged the PR with a couple of commits on top.