add page_builder_class function

[jazzsequence]

would work like post_class does in WordPress core. echoes a generic class (e.g. pagebuilder-part) and takes an optional string parameter to pass specific classes. would be used in markup when building containers/wrappers, e.g.

<div <?php page_builder_class( 'my-class-name' ); ?>>
     <!-- stuff here -->
</div>

which would display as:

<div class="pagebuilder-part my-class-name">
     <!-- stuff here -->
</div>

[jazzsequence]

added in 303459a