[Fix #72] Add support for checker options

Add `option` cell, and `flycheck-def-option-var` macro to declare syntax
checkers.
Print options in checker help
Add option for PHP CodeSniffer coding standard
Document config file vars as buffer local
Update tests to Flake8 message changes
Update channels before installing with PEAR
Declare the docstring argument number
Update the changelog
Add option filter for integer->string conversion
Add flake8 option for maximum complexity