Start in keyword specific step definitions
Add feature and scenario filtering by tags

Add new --tags CLI option to specify how to filter feature and scenarios
to be executed. Several tags can be given as a comma-separated list and
will be ANDed. Multiple occurrences of the CLI arguments will be ORed.

Examples:
  gurke --tags a,b # Will include only features and scenarios tagged
                     with @a AND @b
  gurke --tags a,~b --tags c
    # Will only include features and scenarios tagged with (@a AND NOT
    # @b) OR (@c).

The default for --tags is ~wip.
Improve reporter

* The reporter now prints a step before running it and will colorize it
  according to its status when it's done.