this is the first step in building a new `zola index` command that allows building search indexes as a stand-alone task.

the new Subcommand in src/cli.rs outlines the planned api.

the biggest thing it will be providing compared to current code is optional indexing with tantivy.

this commit doesn't add tantivy or any new functionality other than allowing the existing search indexing to be performed as a subcommand. (passing --index-type tantivy will trigger unimplemented!() panic)

* 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