Add ability to set toolbars for RichTextArea in construct

[mboynes]

We can currently modify the Fieldmanager_RichTextArea toolbars via filter, which works fairly well, but produces a lot of code. It would be nice to allow one to set the toolbars when creating the object, via the options array. For instance (toolbars exaggeratively brief),

$fm = new Fieldmanager_RichTextArea( array(
    'name' => 'my_richtextarea',
    'toolbar1' => array( 'bold', 'italic' ),
    'toolbar2' => array( 'formatselect' ),
    'toolbar3' => array( 'bullist' ),
    'toolbar4' => array( 'numlist' )
) );

We could also setup a few preset toolbar collections. For instance, 'toolbar_preset' => 'basic' might provide a very simple toolbar set, while (the default) 'toolbar_preset' => 'advanced' might provide a more complex one. For usage in the sidebar, 'toolbar_preset' => 'sidebar' could provide an especially narrow set that won't break the sidebar's width. And so on.

[danielbachhuber]

For reference, I went this route for my setup:

$mce_toolbar = 'bold,italic,strikethrough,bullist,numlist,blockquote,add_media,link,unlink,formatselect';
$publication_guidelines = new Fieldmanager_RichTextArea( __( 'Guidelines for Publication' ), array() );
$publication_guidelines->init_options = array(
    'height'     => 50,
    'toolbar1'   => $mce_toolbar,
    'toolbar2'   => '',
    'toolbar3'   => '',
    );