Add support for "lookup latest release in GitHub"

[josh-padnick]

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')):

[brikis98]

The trickiest question is how to handle this in a generic way that would make sense as part of boilerplate core. Looking up GitHub release data, especially for private repos, can get pretty damn messy...

[josh-padnick]

That's a good point. I think a "plugin" architecture that cleanly separates the complexity of github release lookups from boilerplate core is the only way. For example, here's how Vault handles different auth backends:

https://github.com/hashicorp/vault/tree/6beadc1e1c9c7d317ef8074eaa3f26dfcc936f4d/builtin/credential

I'm pretty sure they all implement some common interface. In our case, we could have a plugin concept for "ways of computing a default", define an interface, and then write whatever plugins we want. To get the latest release from GitHub, we could copy a bunch of code from fetch.