Redesign gitignore plugin

- removed code that fetches completions frim gitignore.io each time fish
  is started -- makes logins slow!
- gi.load becomes gi.fish, only contains gi function
- add _update_gi_completions.fish: fetches list of completions from
  gitignore.io and places it into ~/.config/fish/completions
- modify gi function to accept update-completions argument
Add README.md for gi
Merge pull request #175 from jhillyerd/gi-fix