Add "Getting Help and Reporting Bugs" section

The section mentions the mailinglist and points to GitHub's issue
tracker.
New customize group `haskell-decl-scan' and two new flags

This adds the new customize group `haskell-decl-scan' with the two new
flags:

 `haskell-decl-scan-bindings-as-variables'

for controlling whether to put value bindings into the "Variables"
category, and

 `haskell-decl-scan-add-to-menubar'

for controlling whether to add "Declarations" menu entry to menu bar.
This commit also adds the new info manual node

 `(haskell-mode)haskell-decl-scan-mode'

describing the purpose of `haskell-decl-scan-mode'.
Minor clean-up in `haskell-ds-generic-find-next-decl'

This replaces an `(format "%s" (buffer-substring ...))` occurence by
the more appropriate `buffer-substring-no-properties' function.