Summary:
This adds a `CliPlugin` interface and a basic implementation for the
GitHub plugin.
Paired with @decentralion.
Test Plan:
Create a new directory `/tmp/test-instance`, with:
```
// sourcecred.json
{"bundledPlugins": ["sourcecred/github"]}
// config/sourcecred/github/config.json
{"repositories": ["sourcecred/example-github"]}
```
Then, run
```
yarn backend &&
(cd /tmp/test-instance && node "$OLDPWD/bin/sc2.js" load)
```
and observe that the new instance has a cache directory containing a
GitHub database.
wchargin-branch: cli2-load
80c3c38282