vim: use example vimrc from vim runtime files

Rather than keeping our own local vimrc file we can use vimrc_example.vim from
the runtime files distributed with vim. This keeps us up-to-date with changes to
the config when we move to a new vim version.

Also tidy comments around do_install_append so that it is clearer what is
happening.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>