Support for custom generators.

[marcosborges]

Summary

Support for custom generators.

Motivation

I think it would be interesting to customize the generators so we could create our own starters.

Suggestion

One way I think would be cool would be to be able to provide the address of a git repository.

Ex: terraspace new custom NAME https://github.com/my-org/terraspace_my_custom_project_generator

[tongueroo]

Note: Did something similar in the ufo tool. Linking the PR for reference https://github.com/boltops-tools/ufo/pull/27 Think it might help. Will consider PRs for this. No sweat either way of course.