feat(Linkify): Add markdown support

[jadenlemmon]

Summary:
Adds markdown support. I've experimented with several solutions so far and react-markdown got me closest to what I was trying to accomplish. With react-markdown the links are still parsed as before as well as the additional markdown.

Still a WIP.

[jadenlemmon]

One discussion item is how much markdown is supported? Since the initial thought was just links.

[codecov-io]

Codecov Report

Merging #14 into master will decrease coverage by 6.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   74.21%   67.97%   -6.24%     
==========================================
  Files          13       13              
  Lines         159      178      +19     
  Branches       24       27       +3     
==========================================
+ Hits          118      121       +3     
- Misses         30       42      +12     
- Partials       11       15       +4

Impacted Files	Coverage Δ
src/Linkify/Linkify.js	100% <100%> (+5%)	⬆️
src/Dropdown/Dropdown.js	44% <0%> (+2.13%)	⬆️

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[jadenlemmon]

@sappira-inc/engineering
react-markdown has some good control over the markdown by using an allowedTypes prop we can only allow links to start or whatever we choose. lmk thoughts.

[choochootrain]

might as well add a snapshot for markdown rendering and then this is g2g from me

[jadenlemmon]

@choochootrain Added

[choochootrain]

lgtm

not sure about the node types we want to allow but i the skipHtml prop should lock out most of the sketchy parts

[kylealwyn]

Looks like the example is broken. We may need to recursively flatten down Linkify's children.

[jadenlemmon]

@kylealwyn is there a use case for nesting the Linkify component?