Add WP-CLI support.

## SYNOPSIS

```
wp theme check [<theme>] [--format=<format>] [--output=<output>]
```

## OPTIONS

```
[<theme>]
 Theme name. Default: current theme

[--format=<format>]
 Accepted values: text, json. Default: text

[--output=<output>]
 Accepted values: stdout, stderr. Default: stdout
```

## EXAMPLES

```
wp theme check twentyfourteen
```
Should this be `theme-check`? If you wanted that, instead of having a command and a subcommand, you can rename the `function theme` to `function __invoke()`. The latter is a special method