Support for checkboxes

[karllhughes]

I know these aren't standard in all flavors of markdown, but any interest in supporting Github's style of checkboxes in this package?

This example:

- [ ] An unchecked checkbox
- [x] A checked checkbox

Would render as it does on Github:

• An unchecked checkbox
• A checked checkbox

[karllhughes]

Didn't really notice, but this is in the works (possibly) for chjj/marked, so I'm guessing when it's merged in there it will also work in this package, yes?

[vknabel]

@karllhughes Yes. If markedjs/marked#587 will be merged, markdown-to-html-pipe will support checkboxes.

[vknabel]

@karllhughes With [email protected] chjj/marked is now a peer dependency.
That way you can temporarily replace chjj/marked with jhollingworth/marked. Once the PR will be merged and released, even versions like 0.4.0 or 1.0.0 wouldn't require an update to markdown-to-html-pipe.

If there are any other issues or suggestions, feel free to ask.