Allow Corpus to read from multiple notes directories

[wincent]

Use case:

• ~/Documents/Corpus/Personal/
• ~/Documents/Corpus/Work/

All of ~/Documents/Corpus/ can be version controlled, but only Personal can get synced to Dropbox.

The gotcha: need to make it easy to add new notes to the right directory, and to change your mind easily if you do it wrong, and that means coming up with some UI for it.

[wincent]

coming up with some UI for it

Drop-down to select directory, located above the notes list.

[wincent]

Not relevant in Neovim (a form of this already implemented).