The CLI should suggest to require a Node.js package if the backend is not found

[ggrossetie]

The user may not fully understand what is included or not but the CLI can understand the intent and thus suggest a resolution.

For instance:

asciidoctorjs -b blog source.adoc -o dest.html
> Error: unknown backend.
> You might want to require a Node.js package with --require option to support this backend.

We could also suggest to install an "official" backend (please note that this feature depends on #41):

asciidoctorjs -b revealjs source.adoc -o dest.html
> Error: unknown backend.
> You might want to run the following command to support this backend:
> npm install asciidoctor-reveal.js

So if the user requests an "official" backend (without explicitly using the --require option) then we should try to load this "official" backend. If it fails, we should suggest to install the package using npm.