Allow returning hex encoded as an integer

[Richienb]

It would be useful if there was an option that returned the hex as an integer. Such integers are the only type of number accepted by most image manipulation libraries such as omggif and jimp.

// @sindresorhus

[sindresorhus]

You can do Number.parseInt(rgbHex(65, 131, 196), 16). Not sure the use-case is common enough to add a new option for this. If it is, it should be a new method instead, to keep the types simple.