Change the priority to 5 on theme layout filters/actions that devs/users may be overriding on the default priority later.  This makes it easier since they don't need to worry with which priority to use.
Use sanitize_html_class() for sanitizing the theme layouts meta.  Since it's intended to be used as a class, sanitize it as such.
Allow devs to overwrite the theme layouts arguments via a filter hook.
Update inline docs for Theme Layouts.
Code formatting updates for Theme Layouts.
Set an empty string as the default for the $attachment variable. Props @turtlepod See: https://github.com/justintadlock/hybrid-core/pull/63