If we don't use the default docksets, helm-dash will not work smoothly
unless we set the `dash-helm-dash-docset-path' variable when we import
the dash layer(actually, the default dash docset path is always
"~/Library/Application Support/Dash/DocSets").
As reported in #5694 dash layer cannot be configured according to the
documentation. This commit fixes that, and allows you to actually define docset
path with `helm-dash-docset-newpath`. It also defines more sensible default for
docset `~/.docsets`, which is the default path for helm-dash.
Fixes#5694
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".