Closes https://github.com/getzola/zola/issues/732

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

* Mention serve --open flag in CLI documentation

Closes #722

Closes #727

* Update cli-usage.md

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

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

It looks like two links on this page didn't get updated with the change from `./` to `@/` in ec61a57.

Simplify yes/ehh/no links in README.md

0.8.0

Closes #712

Round up reading time

Update actix-web to 1.0

* 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 Dockerfile

Add url for base16 syntax highlighting examples

Fix mistake in `Sorting Subsections` documentation

Add XML declarations to XML templates.

Without this, at least one search engine spider was generating a
validation warning.

Update my example.

* 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