Feat: Add generated id to markdown headings (#833)

[wkillerud]

This PR introduces preliminary support for linking to headings
within an example text using markdown headings.

Since linking in this way requires using the # character it means you
can't link to a section in isolated mode (for instance to
http://localhost:6060/#!/WrappedButton#example-section) on account of
the #! being reserved for routing.

It may still be useful to the reporter of #833 for linking to a section in the list
view, and may act as a base for further improvements 🙂

[wkillerud]

I don't know the inner workings of the RSG router, but if it's possible to rewrite (long term goal for sure) to free the # character using the HTML5 History API it would make this feature much more useful.

Still, even with its limitations this version may add some value :)

[codecov-io]

Codecov Report

Merging #1163 into next will not change coverage.
The diff coverage is 100%.

Impacted Files	Coverage Δ
...arkdown/MarkdownHeading/MarkdownHeadingRenderer.js	100% <100%> (ø)	⬆️

[sapegin]

Thanks for the PR!

We can't use HTML5 History API because it will break external links to Styleguidist and page refresh.

markdown-to-jsx already passes an ID, so we can use it instead of generating our own:

https://github.com/probablyup/markdown-to-jsx/blob/56c99dd95f990e266fca52278f5b394d24fdc6f6/index.js#L998-L1016

[wkillerud]

markdown-to-jsx already passes an ID, so we can use it instead of generating our own

Allright, didn't know that - nice 👍 Made the requested changes and removed the stuff that became redundant.

[sapegin]

Thanks, merged!