Enable Soundcloud player

I've just made a pull request on GitHub to enable Soundcloud player in posts:

• https://github.com/TigerND/steemit.com/commit/5e04c403682b0e881e58e2da1518b5acdef58678
• https://github.com/steemit/steemit.com/pull/90

Guess people really want it:

• https://steemit.com/steem/@ajvest/soundcloud-plugins-for-steem-posts

Here's a sample iframe with Soundcloud audio: