Add the ability to specify request headers when fetching via file

[JamesMcClelland]

Allows headers to be sent when fetching a sound via file, used primarly for sounds stored behind an authenticated endpoint, previously was not possible. Also expandable to allow other headers to be sent to the server

new Pizzicato.Sound({
    source: 'file',
    options: { path: `http://localhost/mysound.mp3`, headers: {Authorization: `Bearer 123456`} }
});