Add support for !avatar and !gravatar in echo.js

[hackerkid]

• Add support for !avatar
• Node tests
• Bugdown tests
• Get bugdown: Fix patterns ordering in Markdown extension. timabbott/zulip#2 merged

Fixes one issue in #1653

[hackerkid]

@timabbott Please check.

[timabbott]

Cool, this looks great, thanks @hackerkid! I merged this after fixing some trailing whitespace. Ideally we'd figure out a way to modify marked.js without needing to patch it, since that's definitely a maintenance liability (as a sidenote, we should probably update marked.js to a current upstream version).

[hackerkid]

@timabbott Should I follow the same approach for !_subscribe_link and !modal_link. I am not sure whether we can do this without patching marked.js.

[timabbott]

Having though about this some more, I think we're going to remove !_subscribe_link as a feature; it's not that good of an experience and we just merged code to have #**StreamName** auto-link to a stream. And !modal_link is just used in the tutorial right now, so not important for local echo.

So I think we're good on that front.