resolves #32 align options with Asciidoctor ecosystem

- use double-hyphen prefix for long options (where possible)
- rename "include-basedir" to just "basedir"
- rename "stylesheetfile" to just "stylesheet"
Restore standard -stylesheetfile doclet option.
Use `-r`/`--require` & `-a`/`--attribute` options, in-line with Asciidoctor.

- support multiple occurrences of these options.
- single `-a` or `-r` option argument may contain multiple values separated by commas
Add docs for --require and --gem-path options.