Add support for "lookup latest release in GitHub"

Many of my boilerplate variables are just a asking what GitHub release / git tag I want from a given repo. The default is the latest version at the time the author wrote the `boilerplate.yml` file so I can never trust that it is in fact the latest.

Therefore, it'd be nice if I could specify some kind of `github-latest-release` plugin in boilerplate that would _compute_ my default value instead of just using a default literal.

Example:

```
What version of module-security should we use? (default: latest release (currently 'v0.1.0')):
```

Start by reading how boilerplate.yml variables and default literals are evaluated, then trace the existing plugin or variable-extension entry points. Define how a github-latest-release value would identify a repository and resolve its latest release or tag, and verify completion using the example prompt and default value described in the issue.

Written by the indexing model from the issue text.