Add support for cancel (or pause?)

[ericpyle]

See recent commit to sindresorhus/electron-dl@cb20484

Since your download manager tries to support resume, it's possible this cancel functionality could double as pause. (Truly canceling may require deleting the what has been downloaded upto the cancel.)

[ndelvalle]

@ericpyle apparently the item is being send as a second argument in the onProgress function, so I think you should be able to call item.cancel(), take a look:

electron-download-manager/index.js

Lines 47 to 49 in 2aec86d

	if (typeof queueItem.onProgress === 'function') {
	queueItem.onProgress(progress, item);
	}

[danielnieto]

Thanks @ndelvalle