Alter plugin discovery

Now that my first pull request got pulled, I figured I'd try another experimental and debatable change.

I tried making hk behave more like Git based on how it is extended. Instead of defining HKPATH (and as a consequence altering the PATH variable temporarily), plugins can now live anywhere in the default search directories and are added if they match "hk-*".

This will provide more options for distributing plugins; making something that depend on a lot of brew formulae? Ship it as a formula! Testing a simple script? Put it in `~/bin`. While we may want to standardize delivery of our stock plugins, I think this flexibility is preferable for all other use cases.