Option to exclude some characters to be decoded

[Jerska]

This is a pull request for issue #18 .

By looking at your rake tasks, I could see performance was an issue for you.
That's why the code has this big if/else, so that it cannot impact current usages of the gem.
I've probably added too many tests, I think you'll choose the ones you think adequate.

Cheers

[threedaymonk]

Sorry for not getting to this sooner.

I think I understand the use case, but I'm not convinced by this as a solution: I don't really like options hashes. What I think I'd like to do instead is to add a Mapping as a first-class concept, so that the interface would be something like:

mapping = HTMLEntities::Mapping::XHTML1.new.without('<', '>')
coder = HTMLEntites.new(mapping)

That might also clean up some of the ambiguity in the SGML mappings. I still haven't quite worked out the interface exactly in my head, however.