Add <field>! method to instances

Calling `start!` on a record that has a `started_on` or `started_at` attribute will set the attribute to now (`Date.today` or `Time.now`). If the record has a boolean `started` field, it will be set to true.

The `past_participle_to_infinitive` method is very basic, but I’m planning to improve it in another pull request to deal with irregular verbs (like `send`).