Strikethrough conversion is incomplete

[tobiaspatton-s4]

markdownToDraft(string, options) includes support for writing markdown with strikethrough text enclosed with ~~ but draftToMarkdown(rawDraftObject, options) does not include support for creating a DraftJS inline style when the ~~ marker is encountered.

It's possible to add a custom entry to BlockStyles when invoking the conversion routine, but since default support is included in one direction, it makes sense to support it in the other direction as well.

I'll create a PR for this issue.

[Rosey]

Thanks for your PR 🙂 merged it and will close this!