Fetch modules from GitHub

[dehall]

Currently the workflow for editing new modules is to create them, download them, then re-upload them. It would be even easier for users if it was possible to fetch modules directly from GitHub, especially branches other than master.

I'm aware of 2 potential ways to fetch modules, there may be more:

1. directly getting the raw JSON, ex GET https://raw.githubusercontent.com/synthetichealth/synthea/master/src/main/resources/modules/appendicitis.json
2. via the GitHub API, ex https://api.github.com/repos/synthetichealth/synthea/contents/src/main/resources/modules

There are rate-limiting concerns, but since there are alternative ways to load the modules I don't see that as a huge issue.

Also note that this request only covers pulling modules, not saving directly to GitHub. I expect pushing would require significantly more work so will be broken out into a separate issue.

[dehall]

implemented in #231