`spacemacs` now handles `rainbow-delimiters-mode` by adding it to the
`prog-mode-hook`, if wanted by the user. Some layers are adding it on
their own mode-hook, having for effect that `rainbow-delimiters-mode` is
called twice, which disable it.
This commit remove these layer-specific definitions of
`rainbow-delimiters` as it is now handled by the `spacemacs`
distribution. It also takes care of running `prog-mode-hook` in modes
that are not derived from it.
Fixes#3902
Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes#2737, because as far as I can tell "+" has no special
meaning in a helm pattern.
Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".