Change plugin folder (xbar)

With BitBar I had a repo of all my plugins nested inside a folder (so I could install dependencies up 1 level) but with xbar there is no option to change the folder. I "solved" this by removing the plugin folder created by xbar and symlinking in my old folder. It would be nice to do this via the GUI in a "supported" way. Thank you!

Temp Fix:

````
rm ~/Library/Application\ Support/xbar/plugins
ln -s ~/git/joshstrange/bitbar-scripts/scripts ~/Library/Application\ Support/xbar/plugins
````

The issue does not name implementation files or tests. Start by locating the xbar GUI entry point and the code that selects the plugins folder, then determine how a user-configurable folder should be exposed. Done means the plugins directory can be changed through the GUI without the symlink workaround and the selected folder is used after restarting xbar.

Written by the indexing model from the issue text.