RFC: Version locking with `antigen lock` command

[desyncr]

In order to be sure which bundle's version the system is running a "version lock" system would record the bundles installation version to be able to later on install the exact same version on any other system.

I propose an antigen lock command to generate a .antigen.lock file, effectively locking-out versions.

Version locking will happen on install and reset with antigen update.

Version locking file location's can be configured with $ANTIGEN_LOCK_FILE, defaults to $ADOTDIR.

Target version v2 or v3.

[desyncr]

This also can go hand-in-hand with our caching mechanism so new users -not aware of caching- can work with Antigen as natural as possible when creating/migrating their configurations, then define a point when they can lock/mark as stable their configs.