Adds support for XDG_CONFIG dirs [WIP]

[cl3wis]

WRT Issue #35

What's done:

• removes default config file set in argparser
• adds method in Config for getting existing config file if arg isn't set

What remains to be done

• support $XDG_DATA_HOME and $XDG_CACHE_HOME
• test (unit tests preferably) the various possible scenarios, if it breaks existing config setups, etc

[mjpost]

This looks good to me. Do you want me to wait for you to do the other items on the TODO list?

[cl3wis]

This code is currently in a working (caveat: tested on a single Linux workstation with no DE installed) state, so there shouldn't be any problem merging it in now. But, if I add the other TODO items first, the changelog could look *so much* nicer... Let me see how much progress I can make this weekend.

[mjpost]

I'll probably just merge this in tomorrow, unless you want another weekend?

[cl3wis]

I guess the data directory can be set easily enough by the user, and I'm not sure that the XDG-specified ~/.local/share/bibsearch/papers offers any significant advantage over ~/.bibsearch/papers, so... maybe it's done?

[davvil]

This a ping for @mjpost and myself (@davvil) to merge this. Somehow I thought it was already taken care of...