Separate interface and base_url for serve

You can now have a different base_url and listening interface.
The `gutenberg serve` cmd now accepts `-u`/`--base-url`, but it defaults to `127.0.0.1`.

Closes #199

* Trigger rebuild when `config.toml` is edited

From #191

Fix #104

Copy only static files that triggered the change notification

Parse _index.md files as sections and render them