Support using custom Drawable as placeholder

[cloudymusic]

I think it would be nice if there was an additional overload of setup() that accepted a Drawable to use as a placeholder background, as an alternative (or in addition) to the background color that's already supported.

In my particular situation, I always know what the first frame of my videos will look like ahead of time, so it could be used to provide a more seamless transition once the video starts playing.

[mklimek]

Added in https://github.com/mklimek/FrameVideoView/commit/29c75966afa8aa6ca25a24360e74f924a8840faf

Enjoy :)

[cloudymusic]

Works perfectly. Thank you! :)