Tag support for bin/pherkin

TODO convert the commandline option to an array of selectors
TODO parse the options into TagSpecScheme for filtering
Converted App::pherkin into a real object to store the tags option
If we've selected tags, ::Parser::parse_file post-filters the scenarios
Support (mostly) for Cucumber-compatible selectors

Convert the commandline tag arguments into a TagSpecScheme list
FIXME this needs a bit of tidying
Tagged digest example
Do the scenario-tag-filtering in the correct place.
Better handling of command line tag options

Refactor tag conversion into its own method.
Add a bunch of tests for that method.
FIXME optimise the output of `_process_tags`
Merged in changes from rjp's Run tagged scenarios pull request