Standardizing tileapi URLs, moving to {{site.tileapi}} rather than hard-coded URLs

The primary purpose of this is to make the docs configurable for different hostnames for [offline](https://github.com/mapbox/offline). Changing `tileapi` in _config.yml will update links and code in current documentation. This also standardizes URLs -- to this point they've been inconsistent in the documentation between `http` & `https`, `a.tiles.mapbox.com` & `api.tiles.mapbox.com`.

Other changes that had to be made: 
* Moved content from YAML front matter (`snippet`) to body in plugins; updated plugins/index.html accordingly
* Removed large blocks of `{% raw %}`/`{% endraw %}` and replaced only where necessary (`{{name}}` in L.mapbox.template) -- this was preventing {{site.tileapi}} from rendering (and 3 instances of {{site.defaultid}} that had slipped through in 1.5.0 documentation); changed generate.js accordingly