Inline validation definition.

  Instead of:

      property :name
      validates presence: true

  ... you can now do:

      property :name, validates: {presence: true}
Allow adding multiple properties.

    eg `property :foo, :bar` or `properties :foo, :bar`

    NB: This was previously possible using undocumented `properties` method but this accepted an
    array. For backwards compatibility, this still works.
Merge branch 'form_syntax' of https://github.com/zubin/reform into inline-validate
make ::property consume one property and ::properties do multiple.
documentation.