[Fix #30] Add Lua checker
Fix sorting of checkers
Fix case (lua -> Lua)
Fix spelling (chance -> change)
Fix process cleanup
Simplify list processing with dash
Fix Emacs Lisp predicate
Remove duplicated validation
Ensure to run cleanup code in the right buffer

Use process buffer as source buffer for *all* post-processing.  Fix
flycheck going into an invalid state if the post-processing occurs while
another buffer is active.
Attach the corresponding error to overlays
Add property to identify all flycheck overlays
Add function to retrieve flycheck overlays
[Fix #27] Echo error messages at point
Fix arguments
Do not show errors if flycheck-mode is disabled
Completely cleanup when disabling flycheck
Release 0.5
Improve tagline and commentary
Require *exact* issue descriptions
Fix recursive customize group