Add additional option that allows for the extension of the markdown parser

[lkuechler]

This PR should allow for custom markdown syntax parsing as requested in this issue: https://github.com/probablyup/markdown-to-jsx/issues/313

I added a new property named additionalParserRules that can be given MarkdownToJSX.Rules which are added at the end of the already existing rules.

[jonotrybe]

I am interested to see this PR merged. We are looking to use markdown-to-jsx for our needs but this would be needed

[quantizor]

Generally fine with this PR, I previously had things private so I could refactor without a major version bump but that hasn't happened in a while so might as well allow the extension.

[Innei]

Hi, Is there any progress here？

[ernests]

Any update on this?

[huksley]

@probablyup Have this been merged or implemented in a different way?