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

* Mention serve --open flag in CLI documentation

Closes #712

* actix_web::fs is now found in the actix-files crate.

* NotFoundHandler is now implemented using ErrorHandlers and the
path to 404.html is provided to the not_found() function using
actix_web::App::data. This setup allows for additional StatusCodes
to be handled in the future, if desired.

* Cleanly serving index.html files for requests to directories now
appears to be possible simply by using index_file("index.html") in
the static files service, so handle_directory() is removed.

* 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

This implementation seems to match the behavior of the previous iron
server.

Static files are rendered as-is, and when a directory is requested, we
attempt to render a `index.html` found inside, or 404 if it's not
present.

The actix docs mention using channels to send a message to the server to
shut it down gracefully while running in another thread (as we're doing
here), but it seems like there would have to be some reorganization in
order to manage this effectively, perhaps holding the channel sender inside
`main.rs` so we can push a message through to the server when the call
to `cmd::serve()` finally returns.

For the time being, I left things without any careful attempts to
cleanup the server thread. This more or less matches the old iron
implementation as far as I can see.

The static file handling in actix is _just_ a little off from what we'd
want.
I left some comments in the source regarding why we can't just use their hook
for directory index redirection.

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`.

Closes #199

From #191

Fix #104

Fix #72

Fix #59