Remarkup rule to embed images

Summary:
Ref T4190. Added the remarkup rule to embed images:

Syntax is as follows:

{image <IMAGE_URL>}

Parameters are also supported, like:
{image uri=<IMAGE_URI>, width=500px, height=200px, alt=picture of a moose, href=google.com}

URLs without a protocol are not supported.

Test Plan: Tested with many of the syntax variations. If the provided URL doesn't point to an image, then a broken image icon will be shown.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T4190

Differential Revision: https://secure.phabricator.com/D16597