Add support to not replace non-ASCII chars not in map

[sobrinho]

[sobrinho]

I'm not sure the best approach here but the idea is that I want to keep all non-ASCII chars as is unless if a replacement is there.

Examples:

I18n.transliterate("João acertou ½ das questões sobre Ω")
#=> "Joao acertou ½ das questoes sobre Ω"

It is possible to add replacements that would replace to itself but then I need to know upfront all the non-ASCII chars I need to keep and that list is quite big.

The implementation can change but the idea is that we would replace only the known non-ASCII chars and keep the others as is.