[EXT] Add `git mind extension remove` subcommand

[flyingrobots]

Problem

Once extension persistence (#261) is implemented, users need a way to unregister extensions they've added. Currently there's no extension remove subcommand.

Work

• Add extensionRemove(cwd, name, opts) to src/cli/commands.js
• Remove the extension name from the persistence config file
• Add case 'remove': to the extension sub-dispatch in bin/git-mind.js
• Update printUsage()

Blocked by: #261 (persistence)

Ref: #252

[flyingrobots]

Resolved in 1e0726d (PR #270)