Add optional generic constraint for FSA type property

[fredrikhr]

• Make Payload type constaint optional (default to undefined) to allow shorthand FSA declarations for Actions without payloads.
• Introduce new option Type type constraint (defaults to string) to allow shorthand FSA declarations for strong typing of the type property.

In TypeScript it is common to define actions with the type property declared as type: typeof ACTION_TYPE.

Thus, we might as well add an additional optional type constraint on the FSA type to express this.

For a Counter application example this makes it possible to change the following code:

import { FSA } from 'flux-standard-action';

const INCREMENT_COUNTER = 'INCREMENT_COUNTER';
type INCREMENT_COUNTER = typeof INCREMENT_COUNTER;
interface IncrementCounterAction extends FSA {
  type: INCREMENT_COUNTER;
}

to this:

import { FSA } from 'flux-standard-action';

const INCREMENT_COUNTER = 'INCREMENT_COUNTER';
type IncrementCounterAction = FSA<undefined, undefined, typeof INCREMENT_COUNTER>;

removing the need to re-declare the type property. The need to explicitly define the Meta type as undefined is unfortunate, but cannot be avoided if backwards compatability is to be maintained.

The type is optional (defaults to string) and is added as a 3^rd constraint and thus maintains backwards compatability.

[JaKXz]

@couven92 can this be closed in favour of #114? I'm seeing a lot of similarities but I'll let you hit the button :)

[fredrikhr]

@JaKXz yes, we can :)