add support for prezto plugins
Test cleanup with two plugins loaded.
Merge branch 'more-tests'
The `-n` in url/dir functions screws up things.

This was messing with `-cleanup`. The fix needs to add blank `echo`s after each
call to `-antigen-get-clone-url` and `-antigen-get-clone-dir` in `-cleanup`
function, or this. I think this is better.
Load `init.zsh` instead of `*.init.zsh`.

This is what prezto seems to be doing, from the source as of today.
Added tests for loading prezto's `init.zsh`.
Formatting.
Added notes on prezto support.