Add xtf::response enumeration

Add xtf::response input formatter
Add xtf::response output formatter

The xtf::response enumeration represents the possible responses to
failures during testing.
Add the -R option to xtf::options
Add response mapping to xtf::environment
Rename xtf::failure to xtf::severity
Rename vars to definitions
Rename variable type to response in xtf::options::options