Part fix of #742

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

* Mention serve --open flag in CLI documentation

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

* 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.

* chore: Update glob to 0.3

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

* chore: Update ws to 0.8

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

Close #693

* 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

Closes #607

Closes #576

Closes #533

Fix #525