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

Fix #204

From #191

And some tweaks as I write the docs

Fix #104

This reverts commit 26e119479c.

Add shortcode for Streamable video

Fix #75

Where all the posts would be serialized