focus mode should use lazygit (if available)

[thorstenhirsch]

Since lazygit can do quite a bit more on a single repo than gitbatch (but it cannot handle multiple repos) I suggest the following feature request:

• on startup check if lazygit is available in $PATH
• if true, use lazygit for focus mode instead of the internal focus mode

I think the workflow will be pretty similar to the current one. You just have to press q instead of tab to return to gitbatch.

PS: If you approve this feature request I think I might be able to implement it myself and send a PR.

[thorstenhirsch]

#131