Add support to flash a product device with firmware

[monkbroc]

I was writing a script to flash product devices and found that our API wrapper didn't support the product flash endpoint.

Manual test:

var Particle = require("particle-api-js");
var particle = new Particle();
var auth = "xyz";
var product = "8178";
var file = '/home/monkbroc/Downloads/bootloader-1.0.0-electron.bin';

particle.flashDevice({ deviceId: '123456', product, files: { file }, auth })
	.then(results => {
		console.log(results.body);
	});

[coveralls]

Coverage increased (+0.04%) to 74.177% when pulling 2dcc4eb on feature/flash-product into d8a27c2 on master.

[coveralls]

Coverage increased (+0.04%) to 74.177% when pulling 2dcc4eb on feature/flash-product into d8a27c2 on master.

[coveralls]

Coverage increased (+0.04%) to 74.177% when pulling 2dcc4eb on feature/flash-product into d8a27c2 on master.

[JamesHagerman]

Looks good!