Assume first dir if only one is configured

[codybuell]

Closes #78

Adds support to cd into a directory if Corpus is not currently in one and only has one configured. Not sure if it makes sense to set num_dirs on init or where it currently is on each call to cmdline_changed. Can move it if you'd like it elsewhere.

[wincent]

Great idea. I think given that cmdline_changed can be called often, it makes sense to cache the value and re-use it. That'll be a small tweak, so I can take it from here.

Thanks for the PR!

[wincent]

Thanks once again for the PR, @codybuell. This is merged now.

In the end, I decided to hide this behind a config flag because running :cd without the user explicitly asking for it might be a bit aggressive, so it's best to make it opt-in.

To activate the behavior — in Vimscript:

let g:CorpusAutoCd=1

In Lua:

vim.g.CorpusAutoCd = 1