* zola init works without argument (defaults to current directory)

* zola init works with existing folders (no non-hidden files permitted)

* Update zola init documentation

* [docs] Optional argument is on a separate line for clarity

* Add tests for init subcommand's respect of existing folders

* Add --open flag to serve command, to open URL in default browser

* Mention serve --open flag in CLI documentation

* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site

This reverts commit 1815155c1d.

Closes #254

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`.

From #191

Fix #104