Closes #773

* 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

* Update cli-usage.md

* clarify that it's only the links from markdown files that are being checked;

* 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.

- unless forced, only use colors in tty
- support CLICOLOR and NO_COLOR vars
- switch to termcolor crate to handle output coloring

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

* Finishing up site

* Make site a bit responsive

* Fix menu responsiveness

* Remove Fira Sans and revamp a bit text

* Update list of syntax and change output of syntax

* Add Rust mention

* Some doc tweaks

* Correct capitalization for GitHub

Correct capitalization for GitHub

* Some CSS tweaks

* More css tweaks + favicon

* Add link to my site

And some tweaks as I write the docs