Wrong support for underline

[Calyhre]

As I said in sstur/react-rte#71, UNDERLINE inline style is not really supported.
Underline does not really exists in markdown, so it makes ++underline++ unusable in other parsers.

[sstur]

I commented on issue 71 in react-rte but basically I agree with you that this should not be the default behavior since it is not "standard" markdown (by which I mean commonmark/gfm).

I do feel that we should accept an option for custom styles like this. I'll create an issue to that effect.