OMZ-bootstrap plugin

**Ready for Merge** (but testing cannot hurt)

This PR adds the `omz-bootstrap` plugin. It is based on @mbauhardt's PR #2178, and includes changes suggested in [this](https://github.com/robbyrussell/oh-my-zsh/pull/2178#issuecomment-38798793) and [that](https://github.com/robbyrussell/oh-my-zsh/pull/2178#issuecomment-38940376) comment. It should make #1035 and #2178 obsolete.

All commands to manage plugins and themes are now bundled into the `omz` function. Some examples:
```bash
omz plugin ls #list plugins (with less)
omz plugin off debian #disable plugin
omz plugin on osx #enable plugin
omz theme set agnoster #enable theme
omz theme get git@github.com:KevinBongart/gitsome.git kevins_theme #download theme
```

This PR also includes completion for the `omz` function.

Fixes #2178