Custom type parsing

[ozydingo]

Suggested call signature:

datetime :create_after, ->(val){Chronic.parse(val)}, **opts

[ozydingo]

We can call some validations around the Proc's call -- can we / should we / how do we validate that the proc returned a reasonable value? Can we make it easy for this use case to specify error conditions / handling / messages?

[ozydingo]

Released 0.3.0 using syntax:

datetime :create_after do |name, val, opts|
  Chronic.parse(val)
end