Add WP-CLI support.

## SYNOPSIS

```
wp theme check [<theme>] [--format=<format>] [--output=<output>]
```

## OPTIONS

```
  [<theme>]
    Theme name. Default: current theme
```

## EXAMPLES

```
wp theme check twentyfourteen

  [--format=<format>]
    Accepted values: text, json. Default: text

  [--output=<output>]
    Accepted values: stdout, stderr. Default: stdout
```