Support 'run_on_start' config option

[olliebennett]

I'd like to be able to avoid running Consistency Fail when Guard starts up. It takes time, and is not important to me until I change the model/schema files. A configuration option for this would be useful. It would still be possible to hit ↵ to manually run everything.

The RuboCop Guard Plugin supports this through all_on_start: false - see implementation.

The RSpec Guard Plugin also supports all_on_start: false (false is actually the default) - see implementation.

It should be quite quick to duplicate this functionality for this plugin. Perhaps the option run_on_start is a better name, because "all" doesn't really make sense for consistency_fail, however.

The (optional) config would become:

guard :consistency_fail, run_on_start: false do
  watch(%r{^app/model/(.+)\.rb})
  watch(%r{^db/schema.rb})
end

Does this sound useful? If so, I can put together a PR.

[ptyagi16]

Sounds good @olliebennett Please send a PR