This adds a bunch of fully integrated test sites.

While here, fix some bugs:

• Fix not being able to read .md or .yaml files instead of .mkd and .yml. .yaml and .md files not found by tack v1.0.0, .yml and .mkd work #22
• Fix title casing. Page variable 'name' is UPPERCASE instead of Title Case #3
• Fix iterate siblings in correct order. Page variable 'siblings' lists pages in lexical order even if page enumeration is used #4
• Fix crash for non-existent asset directories. Crash if public/ folder does not exist #5
• Fix rendering section context. Mustache rendering {{#current}} blocks break page context #8 (Needs Fix building context stack for inner elements of sections cbroglie/mustache#54)
• Fix rendering non-false value sections. Page variables not available as {{.}} inside value sections #23 (Needs Fix rendering non-false value sections cbroglie/mustache#55)

Also, add these features:

• Allow the root index page to be defined with metadata and markup directly in content/. Allow skipping to create an index directory for the root page #19
• Add support for empty pages (no metadata or markup defined) if they have non-empty children. Allow for empty pages when children exist #20
• Add support for children to iterate non-floating child pages. Support children to iterate non-floating child pages #16
• Add support for posts to iterate all child pages marked with a date. Add support for posts to iterate pages marked with a date #18
• Add support for menu sections to iterate all siblings and the current page (only for non-floating pages). Support menu to iterate non-floating siblings and the current page #17
• For each entry of page collections (children, menu, navigation, posts, siblings) add first and last boolean values that can be used to construct better separators. Add first/last flags for page iterators #21