Expose record tags as string constants

[GrzegorzKazana]

Adresses #75 .

Motivation
When interfacing with other libraries/other code, there is sometimes a need for for using the tags from the tagged union. This results in need to define our unions as follows, which is unnecessarily verbose and boilerplateful.

const ON = 'ON';
const OFF = 'OFF';

const State = unionized({
  [ON]: ofType<{}>(),
  [OFF]: ofType<{}>()
})

// do something later with tag
console.log(ON)  // 'ON'

Solution
Expose record tags as a property on the Unionized object.

const State = unionized({
  ON: ofType<{}>(),
  OFF: ofType<{}>()
})

// do something later with tag
console.log(State.tags.ON)  // 'ON'

/*
State.tags is:
{
  ON: 'ON';
  OFF: 'OFF';
}
*/

[coveralls]

Coverage remained the same at 100.0% when pulling 046b830 on GrzegorzKazana:expose-tag-record into 55240dd on pelotom:master.