Use strings for all flag operations and comparisons.
Use strings for all flag operations and comparisons.

This resolves an issue reported in gh-9, where calling `flag.length`
caused an error in Ruby 1.8.7 due to symbols lacking a `#length` method.

Although Ruby 1.8.7 has been retired, this change also has the added
benefit of a extremely small speed improvement due to fewer
Symbol <> String conversions, with no downsides that are immediately
apparent to me.

closes gh-9

dalehamel/ruby-pandoc,alphabetum/pandoc-ruby,alphabetum/pandoc-ruby