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.

Also use Option for Page::summary

Close #38

Copy only static files that triggered the change notification

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

Using a greedy regex could lead to unintended consequences, like the
inability to syntax highlight a unified diff. This resolves issues like
that by using .*? instead of .* so that the closing +++ is matched as
soon as possible.

Parse _index.md files as sections and render them