add option to default hide media

[braid-sync]

From daiyi:

It would be pretty sweet if we could hide media, especially ones that move. They're really distracting, especially when there as multiple moving things across the screen. I think it would be in line with your mission to increase productivity too.

[braid-sync]

➤ Rafal Dittwald commented:

I think we could implement similar to how some sites deal with youtube / twitter / flash etc. embeds... ie. "click to show/play"