Allows setting of the --continue flag with flog

This is useful to me, because the version of ruby_parser in use by flog
does not support ruby 2.0's keyword arguments. Passing the --continue
flag allows metric_fu's flog to run to completion. Then saikuro barfs,
but hey, at least flog runs.

This can then be set in the .metrics file with:

    MetricFu::Configuration.run do |config|
      config.flog = config.flog.merge({continue: true})
    end