Add churn option (--ignore_files)

[such]

Allow using --ignore_files option for churn

config.configure_metric(:churn) do |churn|
    churn.ignore_files = %q("Gemfile, Gemfile.lock, config/locales/fr.yml, config/locales/en.yml, db/schema.rb")
end

[bf4]

If you're up for it, this looks like a great opportunity to begin work on #167 which is basically to not require code changes for most of a metric's run options, but just to enable passing them through.

For example

:ignore_files => %q("Gemfile, Gemfile.lock, config/locales/fr.yml, config/locales/en.yml, db/schema.rb")

would map to the command-line argument

--ignore_files Gemfile,Gemfile.lock,config/locales/fr.yml,config/locales/en.yml,db/schema.rb

If you chose to do this, please create the options hash with the Map gem, instead of a the hash primitive. (It's included in metric_fu via churn)