Restructure Replacers into groups so it is easier to create custom IScheme implementations

[baynezy]

In #106 a user wants to not have HTML entities replaced. This is an expected behaviour, but it makes sense that some people would want to ommit this behaviour. This can already be achieved by making a custom implementation of IScheme and passing it to Convert at construction time. However, this would require duplicating a lot of code.

IReplacers should be grouped making it easier to create new IScheme implementations with a small amount of code.