add support for ondir

[shakeelmohamed]

https://github.com/alecthomas/ondir

The main issue is ondir isn't distributed on brew or cask so it'll have to be built during setup - then getting stow to do the right thing is a bit tricky.
I'd want to change $SPLUNK_HOME and $SPLUNK_SOURCE based on $(pwd).

[shakeelmohamed]

Maybe direnv is actually better (MIT license & pure bash): https://github.com/direnv/direnv

More discussion on StackOverflow: http://stackoverflow.com/questions/5848832/directory-based-environment-variable-scope-how-to-implement/38482443