Create flag-compatible infrastructure

Adds infrastructure to become flag-compatible with existing rustdoc.
There are a few general buckets a flag falls into:

1. Flag stays the same
2. Flag is no longer supported in the new rustdoc
3. Flag has moved (usually under a subcommand)

Most flags have still not been implemented and they will say so. Also,
some flags are in a limbo state where they are probably going to be
unsupported in the future. Until that decision has been declared, they
are marked as unimplemented.

BREAKING: This change moves `-o` from open flag to output flag.

src/main.rs

@@ -188,6 +188,7 @@ fn run() -> Result<()> {
 
     match matches.subcommand() {
         ("build", Some(matches)) => {
+
             // FIXME: Workaround for clap #1056. Use `.default_value()` once the issue is fixed.
             let artifacts: Vec<&str> = matches
                 .values_of("artifacts")