Re-define highlights on ColorScheme autocommand

[blueyed]

When using :colorscheme highlights are deleted, and need to be
re-defined.

This wraps/moves this into semshi#init(), and triggers it on the
ColorScheme autocommand.

It also wraps this and the conditional definition of the other
autocommands in a new SemshiInit augroup, to not define the
autocommands multiple times.

Fixes #70.

[DanielArndt]

Thanks @blueyed -- I tested this and it does fix my issue as reported in #90 (comment) (which would have been more appropriately reported as part of #70).

[Stealthii]

Can confirm this fix handles updates to syntax/colorscheme after load.

To test this PR's changes with VimPlug:

"Plug 'numirias/semshi', { 'do': ':UpdateRemotePlugins' }
Plug 'blueyed/semshi', { 'branch': 'handle-ColorScheme', 'do': ':UpdateRemotePlugins' }