WET theme: Added ability to include/exclude various page sections using front matter

Adds the ability to include/exclude the following through front matter in the .hbs files (affects mobile panel as well):

Include:
* Secondary menu ("secondarymenu": "true")

Exclude:
* Language toggle ("languagetoggle": "false")
* Mobile panel ("mobimenu": "false")
* Search ("sitesearch": "false")
* Site menu ("sitemenu": "false")
* Breadcrumb ("breadcrumb": "false")
* Site information/footer ("footer": "false")

Note: Parameter names match the existing include file names (in case anyone is wondering about the naming convention).