This increases performance of git prompt filter by fetching branch name directly from `.git/HEAD` file instead of spawning `git` process. Apart from increasing performance this may also solve potential problems with `git` output, as reported in https://github.com/cmderdev/cmder/issues/692
This implements custom logic to detect if we're inside of working tree
and skips os calls if not. This will improve performance of prompt
handling, especially for mercurial prompt.
* Adds package to sources.json
* Merges all cmder's lua modules into one -> cmder.lua to exclude future conflicts
* Adds external modules loading logic
