inclusion support

[Ana06]

The following Rails code:

# User model
validates :state, inclusion: { in: ['unconfirmed', 'confirmed', 'locked', 'deleted', 'subaccount'] }

# user edit view
= f.text_field :state

can be translated into the following html code:

<input id="user_state" name="user[state]" type="text" pattern="(unconfirmed)|(confirmed)|(locked)|(deleted)|(subaccount)"/>

Can I send a PR?

Related to #23