Add Download methods

[celestix]

Add Download* methods to download various available media, use td/telegram/downloader for the purpose.

[IndrajeethY]

Ok

[TrixiS]

Any updates?

[celestix]

Any updates?

We're working on a common method for it, it should be out soon. Thank you for the patience!

[celestix]

@TrixiS I have pushed the progress f75a0aa

You can try it if you want (It's untested from our side).