Introduce new checker definition macro

Use RX for error patterns, and reduce quoting
Add syntax checker for LESS
Update changelog
[Fix #160] Merge branch 'purcell-lesscss'

* purcell-lesscss:
  Update changelog
  Add syntax checker for LESS
Merge branch 'master' into checker-definitions

* master:
  Update changelog
  Add syntax checker for LESS
Define LESS checker
Simplify LESS patterns

Remove cruft targeted at previous less versions
Use the (message) form exclusively
Introduce a (file-name) form
Document translation functions
Fix docstring of -declare-checker
Add comprehensive docstring for -define-checker
Slight docstring fix
Document -define-checker in the manual
Port examples to -define-checker
Remove a useless warning

The whole -declare-checker macro is obsolete, so it doesn't make any
sense to mark a single feature thereof as uber-obsolete.
Document that errors are signaled
Fix -error-pattern-p test
Fix Texinfo errors and warnings
Update changelog