External plugin support

**Is your feature request related to a problem? Please describe.**
I currently work at AWS and we use a couple of internal tools that make our workflow a little unique. We have a bunch of Git repositories in our directory and make pull requests that can involve multiple repositories at once. Sometimes I have to exit out of the current repository, then `cd` to a different repository.

**Describe the solution you'd like**
Custom plugin support so that a new window or something of sorts can show up so we can integrate LazyGit to Amazon's internal build tools.

**Describe alternatives you've considered**
`cd`ing to different repositories