Ability to auto-file PR to update foyer docs with new FF repo

[ctk3b]

• User creates new forcefield repo from this template
• User enables travis for their repo
• User tags release
  • If tests succeed, file a PR agains the foyer repo that adds a link to "force fields using foyer"

[chrisiacovella]

We should probably have a completely separate repo for cataloging forcefields others have created. We also should define some best practices, like if you want to submit to the catalog you need to tag a version (so if you use the forcefield, it's immediately clear which version). Since this is all done in git, we can of course take advantage of history (e.g., maybe v1 is associated with a certain paper...v2 has some other optimizations for specific cases).