Add a WordPress Code Editor field type

[garretmh]

WordPress 4.9 added the Code Editor API (https://make.wordpress.org/core/2017/10/22/code-editing-improvements-in-wordpress-4-9/) to generate CodeMirror editors. It requires a bit of JavaScript hookup but would allow for much nicer WordPress-official alternative to textarea_code.

[jtsternberg]

This is a great idea.

[jtsternberg]

This should probably replace the default behavior for the textarea_code field type, with an options opt-out, e.g. 'options' => array( 'formatted' => false ).