Update markdown parsing library to github.com/gomarkdown/markdown (#944)

vendor/github.com/gomarkdown/markdown/changes-from-blackfriday.md

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+## Changes from blackfriday
+
+This library is derived from blackfriday library. Here's a list of changes.
+
+**Redesigned API**
+
+- split into 3 separate packages: ast, parser and html (for html renderer). This makes the API more manageable. It also separates e.g. parser option from renderer options
+- changed how AST node is represented from union-like representation (manually keeping track of the type of the node) to using interface{} (which is a Go way to combine an arbitrary value with its type)
+
+**Allow re-using most of html renderer logic**
+
+You can implement your own renderer by implementing `Renderer` interface.
+
+Implementing a full renderer is a lot of work and often you just want to tweak html rendering of few node typs.
+
+I've added a way to hook `Renderer.Render` function in html renderer with a custom function that can take over rendering of specific nodes.
+
+I use it myself to do syntax-highlighting of code snippets.
+
+**Speed up go test**
+
+Running `go test` was really slow (17 secs) because it did a poor man's version of fuzzing by feeding the parser all subsets of test strings in order to find panics
+due to incorrect parsing logic.
+
+I've moved that logic to `cmd/crashtest`, so that it can be run on CI but not slow down regular development.
+
+Now `go test` is blazing fast.