Use new function `_bash_it_homebrew_check()` in existing plugins and completions which look for Homebrew.
Alsö, use `$OSTYPE` instead of calling external `uname` binary.
* lib/helpers: use `$OSTYPE` instead of `$(uname)`
* plugins/osx: use `$OSTYPE` instead of `$(uname)`
* plugins/boot2docker: use `$OSTYPE` instead of `$(uname)`
* plugins/python: use `$OSTYPE` instead of `$(uname)`
* plugins/base: use `$OSTYPE` instead of `$(uname)`
Alsö, use `[[` instead of `[` as the former has less insane argument handling being shell syntax rather than a builtin command that must emulate being a real binary
* completion/brew: use `$OSTYPE` instead of `$(uname)`
* completion/git: use `$OSTYPE` instead of `$(uname)`
Alsö, use `[[` instead of `[`.
* completion/fabric: use `$OSTYPE` instead of `uname`
* theme/demula: use `$OSTYPE` instead of `$(uname)`
* theme/rana: use `$OSTYPE` instead of `$(uname)`
- General cleanup of system completion
- Use homebrew's completion loading script instead
- Deprecate completions covered by system
- Adjust load priority of brew to happen after system