Feature: Add flag to format unix timestamps into Javascript Date timestamps

[jordond]

Right now the response that comes back from DarkSky will not create the correct date in javascript. You have to do this:

const { dt } = forecast.current
const data = new Date(dt * 1000) 

It would be nice to have an optional flag that would convert the response automatically.

Something like:

new DarkSky('token', { formatTimestamp: true })

[nschild]

or even better, provide a wrapper function for dates:

new DarkSky('token', { formatTimestamp: (timestamp) => timestamp * 1000 })

new DarkSky('token', { formatTimestamp: (timestamp) => new Date(timestamp * 1000) })

new DarkSky('token', { formatTimestamp: (timestamp) => moment(timestamp * 1000).format('h:mma') })

[jordond]

Closing because of #385