Add setting to not automatically expand images, gists, embeds, etc. (keep collapsed)

Some people dislike Gitter’s feature of automatically inlining and expanding gists, videos, and other things that cause fancy embeds. It would be nice to have an option to not automatically expand these.

• @sadmac7000: https://gitter.im/ceylon/user?at=553dd491bbc455106370c1a7
• @jvasileff: https://gitter.im/ceylon/user?at=554b7c2fa5eca5f3705b7391
• @sadmac7000: https://gitter.im/ceylon/user?at=554fa224629c6099462539b0
• @quintesse: https://gitter.im/ceylon/user?at=55508082cb59bb0b439660d1
• @lucaswerkmeister: https://gitter.im/ceylon/user?at=5564b60a0d9fc84f46ba3f8d
• @sadmac7000: https://gitter.im/ceylon/user?at=556c178b9f3a9b7b060a155e
• @sadmac7000: https://gitter.im/ceylon/user?at=556e3e70777c17d06a13aa9f
• @tombentley: https://gitter.im/ceylon/user?at=557199ac05c872ce6ac807cf