Improve formatting of documentation
[Fix #179] Highlight error symbols by default

Error sexps were way too slow for some modes.
Improve error navigation

Navigate errors by looking for overlays, instead of filtering the error
list.  After all, overlays are what the user sees.
Merge branch 'symbol-highlighting'

* symbol-highlighting:
  Improve error navigation
  [Fix #179] Highlight error symbols by default
  Improve formatting of documentation
Fix universal prefix arg to flycheck-next-error