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
```
`WP_CLI::error()` will exit for you with the proper error code.