Add StripMarkdown option (irc). (#1145)

Enable `StripMarkdown` to strip markdown for irc.

vendor/github.com/writeas/go-strip-markdown/README.md

@@ -0,0 +1,51 @@
+# go-strip-markdown
+
+[![GoDoc](https://godoc.org/github.com/writeas/go-strip-markdown?status.svg)](https://godoc.org/github.com/writeas/go-strip-markdown)
+
+A Markdown stripper written in Go (golang).
+
+## Usage
+You could create a simple command-line utility:
+
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/writeas/go-strip-markdown"
+	"os"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		os.Exit(1)
+	}
+	fmt.Println(stripmd.Strip(os.Args[1]))
+}
+```
+
+You could pass it Markdown and get pure, beauteous text in return:
+
+```bash
+./strip "# A Tale of Text Formatting
+
+_One fateful day_ a developer was presented with [Markdown](https://daringfireball.net/projects/markdown/).
+And they wanted **none of it**."
+
+# A Tale of Text Formatting
+#
+# One fateful day a developer was presented with Markdown.
+# And they wanted none of it.
+```
+
+## Inspiration
+This was largely based off of [remove-markdown](https://github.com/stiang/remove-markdown), a Markdown stripper written in Javascript.
+
+## Used by
+
+This library is used in these projects:
+
+* [WriteFreely](https://github.com/writeas/writefreely)
+
+## License
+MIT.