Make it easier to specify partial types

[tonyxiao]

There's this amazing tool to generate io-ts from json - https://transform.now.sh/json-to-io-ts

However it doesn't work with optional attributes. The current way (https://github.com/gcanti/io-ts#mixing-required-and-optional-props) of specifying partial is really complex / cumbersome. Could we support syntax like

const MyType = t.type({
  requiredField: t.number,
  optionalField: t.optional(t.string)
})

[gcanti]

I wish... (see #140)

[tonyxiao]

awww :( That's too bad. Any workaround you would recommend?