Add a new post check ('match' keyword) which check the value(s) match the given
regular expression.
Create a 'print_error' method for printing usage and an error message when post
checks don't pass.
Remove dependency to PyYAML (see 'https://pypi.python.org/pypi/yamlordereddictloader').
Replace calls to 'eval' by builtins functions from 'sys' module.
Some coding style changes.