Allow providing descriptions and other metadata for ports

[bergie]

It would be useful to enable providing additional metadata about ports when declaring them for components.

Some use cases:

• Description on what the port does when it receives data port descriptions noflo-ui#34
• More information on data types Packet/port datatype information? #84
• Whether data to a port is required or optional Required/optional ports meemoo/dataflow#50

This could look something like:

    @inPorts =
      in: new noflo.Port 'object',
        description: 'Containing element to use for limiting element queries'
        required: false
        interface: Element
      selector: new noflo.Port 'string',
        description: 'CSS query for getting the element'
        required: true

Somewhat also related to #98.

[forresto]

Also good for reducing if @outPorts.x.isAttached() boilerplate.