Add feature to download mp4 videos or gifs from subreddit (#1103)

* reddit mp4 and gif,audio for mp4 file is not implemented yet

* add audio support

* change some regular expresion,less string parsing code

* fix up and README update

fix up and README update

fix up

* feature:for mp4 video can choose from 5 different resolution

* feature: image download support

* fixup image download & different resolution suppor

fixup image download & different res download

* simplify the code

Co-authored-by: Xinzhao Xu <z2d@jifangcheng.com>