Add Freeze support

• The current hab uri is stored in the HAB_URI environment variable.
• Ability to freeze current hab configuration. The current configuration is stored in the HAB_FREEZE environment variable.
• Ability to restore a frozen hab configuration in a new session even if the hab configuration was changed after freezing. This can even work to restore a configuration on another operating system than it was frozen on if properly configured.
• CLI no longer requires passing a URI where it will be ignored.

What's Changed

• [pre-commit.ci] pre-commit autoupdate by @pre-commit-ci in #11
• [pre-commit.ci] pre-commit autoupdate by @pre-commit-ci in #12
• Add freeze support by @MHendricks in #13
• Update pre-commit for setup-cfg-fmt preserving py 3.6 support by @MHendricks in #15

Full Changelog: 0.6.0...0.7.0