Support for Launchy

[RobinDaugherty]

This PR relies on the changes that are in #19, so that PR must be merged before this one.

Add support for the launchy option, which mirrors behavior that the guard-rspec plugin supports. For example:

guard :rubocop, cli: %w(--format fuubar --format html -o ./tmp/rubocop_results.html), launchy: './tmp/rubocop_results.html' do
  # ...
end

This will cause the Rubocop results to be opened in the default browser when the guard runs, in addition to being output in the terminal.

[coveralls]

Coverage remained the same at 100.0% when pulling 736d49f on Ahalogy:launchy into dffc18d on yujinakayama:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 11a85a9 on Ahalogy:launchy into dffc18d on yujinakayama:master.

[RobinDaugherty]

To satisfy rubocop running on Travis CI (the only reason for the build failure) the following changes need to be made:

• PR Conform to Ruby style guide on %r #17 or something equivalent. At this time, the changes in that are suspect (is that regex really equivalent, or is it fixing a bug?) but the syntax change needs to be made.
• Refactoring the Runner class to make it shorter. I think the way that guard-rubocop does it is probably the right way: adding a Notifier class. It might also make sense to add an Options class as well.

I'm hoping to make these changes, but I probably won't be able to do it soon.

[yujinakayama]

Merged via #35. Thanks!