spacemacs/add-flycheck-hook was not really hooking anything, change the name
to better reflect what it does.
Also changed the push for a add-to-list to avoid duplicates.
- Add option to disable by default
- Use local-vars hook to allow per-project enable/disable
- Don’t enable helm-gtags-mode (no need)
- Move emacs bindings from helm-gtags-mode-map to ggtags-mode-map
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".
