Fix lang switcher

This MR adds a simple plugin so the Pug templates know what is the current page, and with that it changes the language switcher to stay on the same page.

• 7aa324e0 - layouts: remove unused get_tools mixin
• 445fb423 - layouts: remove unused get_lang_switcher mixin
• 9e8e4a74 - layouts: Reformat helper functions
• ac43c075 - Create directory for metalsmith plugins
• 4b720614 - metalsmith: Add new custom plugin to keep track of filenames in metadata
• 90f621a5 - metalsmith: Avoid including extra files from static/
• 3c44fbb7 - layouts: Stay on the same page when switching lang
• b9e89a05 - layouts: Add get_page_slug mixin to fix filename handlings bugs

Fixes: #229 (closed)