Allow a direct "warnings as errors" flag

[jcollins-g]

This flag should cause dartdoc to treat all warnings as errors for the purposes of returning an exit code for CI.

Some questions:

1. Do we want to still internally track as warnings vs errors and change the exit code only? Or simply convert to errors.
2. Do we want to allow packages to individually opt out via dartdoc_options.yaml, and how does the flag interact with the other warning control flags?

@mit-mit

[jcollins-g]

$ dart help analyze
Analyze Dart code in a directory.

Usage: dart analyze [arguments] [<directory>]
-h, --help                   Print this usage information.
    --fatal-infos            Treat info level issues as fatal.
    --[no-]fatal-warnings    Treat warning level issues as fatal.
                             (defaults to on)

as @mit-mit suggested, aligning with dart analyze here seems like a good idea.