Install on OSX fixed

- Makefile relied on XDG_CONFIG_HOME environment variable, which doesn't
  exist on OSX
- Assumes fish config dir is ~/.config/fish
Tab completion fixed for OSX.

Completion script relied on GNU sed. OSX ships with BSD sed, which
doesn’t interpret escape sequences.
Update README
Update README
Update README.md
Merge pull request #1 from mikebobroski/master

OSX compatibility + New Completion form