Manage CLI extensions

## Usage

`circleci extension <command> [flags]`

## Targeted Commands

| Command   | Description                   |
| --------- | ----------------------------- |
| `install` | Install an extension          |
| `remove`  | Remove an installed extension |

## Flags

Global flags: `-c, --config`, `--debug`, `--no-color`, `-q, --quiet` — see `circleci --help`.

## Details

Manage CircleCI CLI extensions.

Extensions are binaries named circleci-<name> that add new
commands to the CLI. Once installed, an extension is invoked transparently
as 'circleci <name>'.

Use 'circleci extension install <name>' to fetch an extension from the CircleCI
extension registry and verify its checksum before installing it.

