Make SOURCE_DIR configurable

[samatjain]

I don't like keeping my repository in ~/.local/etc/, especially since I have my own conventions for storing repositories that I frequently update.

SOURCE_DIR should be able to be set from both the command-line, and be read from a configuration file. (e.g. ~/.config/dots/source_dir)

[evanpurkhiser]

I like this idea, and I know I considered it at some point after the fact. I think the reason I never got around to it was because it would require a bit of refactoring since the it's just a constant global right now.

[runiq]

I'll take a shot at this today if that's alright with you.

[evanpurkhiser]

Absolutely, feel free!