Add ability to link untracked files

[andsens]

PR #113 brought up the issue of being able to link untracked files. Here are two use cases:

The motivation for this is that I have some dotfiles that are untracked by git since they contain private information but which I still want to link. Also, when I'm working on my emacs configuration I sometimes have new files that I don't want to track right away but there's no way to link them easily since they're inside a submodule.

[andresilva]

Hey! Now that issue #108 is fixed would you consider merging this feature if I were to update my implementation (i.e. PR #113)? This is really important for me and currently I'm using my own fork of homeshick with this implemented.

[andsens]

Yeah, totally. Go right ahead. I'll review and merge :-)

[andsens]

Oh, you'll probably have a bit more of a challenge implementing it this time around. I had to factor the listing of submodule files out into an executable script because git submodule foreach executes code in a weird way before 2.0. I think you'll have to either add a parameter to submodule-files.sh or export a variable. Sorry :-/

Alternatively: If you can figure out how to get the code in submodule-files.sh back into the get_repo_files() function again, that'd be even better :-)