Make driver::driver::parse_cfgspecs parse any variety of meta_item

[catamorphism]

A FIXME in parse_cfgspecs says:

    // FIXME: It would be nice to use the parser to parse all varieties of
    // meta_item here. At the moment we just support the meta_word variant.

[lucab]

While looking into it, I got the impression that a Parser would need a full-blown Session, which we don't have yet as we are just building its options...

[brson]

Parsers do not require a Session, only a parse_sess. The best way to create a parser here is probably with new_parser_from_source_str: https://github.com/mozilla/rust/blob/incoming/src/libsyntax/parse.rs#L183