Subordinate v6.0.0-alpha.0

• 'use strict' in unit test.
• Rename Permate varaible to subordinate.
• Build with Node.js 12,14, and 15 on CI/CD.
• Build with Actions, ship coverage to Codecov.
• Update LICENSE for 2020.
• Add .gitgnore and .npmignore.
• Update package.json with shorthand URL.
• Update with new subset function from Rescue.

Closes #45.