Add support for cancel (or pause?)

[ericpyle]

See recent commit to https://github.com/sindresorhus/electron-dl/commit/cb20484c2d3c9e2ecdff40cc6e81a8afac51353e

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:

https://github.com/danielnieto/electron-download-manager/blob/2aec86d97a0cd7a75a94921524028992a43dbebb/index.js#L47-L49

[danielnieto]

Thanks @ndelvalle