Intuitive window splitting keybindings
Reformat doc for split window key bindings
Fixes #270 Evil keybindings no longer work in *Scratch* buffer
Merge github and git layers

Github support is enabled with `git-enable-github-support`
variable, to be put in `dotspacemacs/init`
Update git layer readme toc
Remap `g c` to `g h` in git layer

And better consistency with the other `highlight` clear actions.
contrib(git): add github-browse-file

* SPC g c b
Merge branch 'develop' (v0.33.0)