Add gatsby-remark-autolink-headers

[tmeasday]

Solves chromaui/learnstorybook.com#347 very nicely indeed!

@jonniebigodes -- note that the links (in the source markdown) need to have a trailing .md (before the #) like I POC-ed in this branch

Note the link changed there in storybookjs/storybook@dcff96b is the "custom webpack configuration" link. Note that it works both:

1. On GH directly: https://github.com/storybookjs/storybook/blob/test-update-link/docs/configure/overview.md
2. In development gatsby (on this branch if you have that branch checked out in the monorepo).
3. In built gatsby (on this branch similarly).

Note there is some kind of negative interaction between our styles and those of gatsby-remark-autolink-headers -- there are lots of options to customize this -- I will leave that up to others ;)

[jonniebigodes]

i was checking the pr and with that i kinda had a homer simpson moment (pardon the bad pun). I checked locally the next branch and looked at the gatsby-config.js file and saw that the .md extensions will be stripped down.

Styling wise the plugin doesn't offer much possibilities. We have a way to include icons and assign a classname to the header and that's it. This is something that needs to be checked a bit more thoroughly.

[tmeasday]

@domyen lmk what styling you want for these auto links

[kylesuss]

Done in #126 w/ styling as well. Thanks!