Updates underline to use ins tag (via ++)

[codenamev]

Markdown will underline ins text by default, and bolds __.

[jpuri]

Hi @codenamev ,

You can use config object to over-ride the defaults provided by the lib: https://github.com/jpuri/draftjs-to-markdown/blob/master/js/index.js#L553

I will add this more clearly to readme.

[WalissonRodrigo]

You can use my version that already solves this problem of compatibility with the library just by adding new properties to your config by invoking the standard draftToMarkdown function.
LINK PULL REQUEST