Fix #59

Also add a Site::render_section method

Useful if you're creating a section only to access it in the index but
do not want a section page for it

Closes #14, #43

Avoid issues with temp files from some editors

Also fix gutenberg serve that has been broken (not
re-rendering content) for quite some time now.

Copy only static files that triggered the change notification

To every templates, useful when doing
navigation to know which link is currently
active

Parse _index.md files as sections and render them