[vim] Move statusline autocmds to augroup

Autocommand groups (augroups) kind of work like namespaces, and are
useful for organization and debugging. The autocmds related to changing
the color of the statusline according to the current mode, should
therefor be placed in an augroup.

More information:
- https://gist.github.com/romainl/9ecd7b09a693816997ba#file-vimrc-L150