Allow remote references to .taskcluster.yml files processed by Taskcluster-GitHub

I've pushed a few changes that I think address all of the comments above. Here's a quick summary:

Adjust config-from format to be GitHub-specific, and explicitly allow tag, branch, and revision references
Add additional context that will make .taskcluster.yml files aware of the repository and revision of the config-from file at runtime. (This will allow them to, eg: include this information as additional metadata.)
Clarifications around merging and precedence of context from various sources
Clarifications around what is supported in project repo .taskcluster.yml files vs. config-from .taskcluster.yml files.

taskcluster / taskcluster-rfcs