inline <script> tag sanitization, & section opt() enhancement. fixes #330

The primary aim of these changes is to prevent inline scripts as _plain-text_ (shortcodes are unaffected) from being allowed from widgets, or section option inputs (including real-time protection in _data-sync_) which can break access to the editor. 

This is more of a problem with section option inputs, as if the editor is broken, the problem becomes much more complicated to fix.  Hence the modification of the `opt()` method detailed below. This does not affect scripts added though shortcodes or any other means.

In addition to this, the `PageLinesSection->opt()` method has been improved and a filter `pl_section_opt` has been added to the return value (which is used with the new `pl_clean_scripts` function.  Also, falsy values such as `0` can now be used as a default value, rather than it returning just `false`.