Improve support for MacOS X directory icon (update_terminal_cwd)
Improve support for MacOS X directory icon (update_terminal_cwd)

Improvements:
- check if update_terminal_cwd is available instead of relying on the
  kernel version
- test the feature once at init time instead of at prompt time

Note that zsh is not yet supported for that feature unless the
update_terminal_cwd has been imported in your environment.
I think it would be possible to support it, but I don't have access to
MacOS X machines, so... patches are welcome!

joshuarubin/dotfiles,illwieckz/liquidprompt,joshuarubin/dotfiles,nojhan/liquidprompt,jonathanjsimon/liquidprompt,colinlieberman/liquidprompt,laurentbardin/liquidprompt,jaesivsm/liquidprompt,dan-cleinmark/liquidprompt,alexprengere/liquidprompt,timhyson/liquidprompt,BobHood/liquidprompt,milouse/liquidprompt,tkuminecz/liquidprompt,lyoshenka/liquidprompt,pkkolos/liquidprompt,dpilone/liquidprompt,kenrachynski/liquidprompt,elementalvoid/liquidprompt,joshuarubin/dotfiles,smadhueagle/liquidprompt,tsbarnes/liquidprompt,dtschan/liquidprompt,tstone2077/liquidprompt,joshuarubin/dotfiles,mathieumd/liquidprompt,tocomo/liquidprompt,enrdstrm/liquidprompt