Add Task Lists

Implement task lists.

Fixes #67

README.md

@@ -27,7 +27,7 @@ Mmark adds the following syntax elements to [black friday](https://github.com/ru
 
 * TOML titleblock.
 * Including other files.
-* More enumerated lists.
+* More enumerated lists and task-lists.
 * Table and codeblock captions.
 * Quote attribution (quote "captions").
 * Table footers, header and block tables.

block_test.go

@@ -635,6 +635,11 @@ II.  or2`,
 * un1
 * un2`,
 		"<ol>\n<li>or1</li>\n<li>or2</li>\n</ol>\n\n<ul>\n<li>un1</li>\n<li>un2</li>\n</ul>\n",
+
+		`- [ ] This is an incomplete task.
+- [x] This is done.
+`,
+		"<ul>\n<li><input type=\"checkbox\" disabled=\"\"> This is an incomplete task.</li>\n<li><input type=\"checkbox\" checked=\"\" disabled=\"\"> This is done.</li>\n</ul>\n",
 	}
 	doTestsBlock(t, tests, 0)
 }

html.go

@@ -602,6 +602,14 @@ func (options *html) ListItem(out *bytes.Buffer, text []byte, flags int) {
 		out.WriteString("</dt>")
 		return
 	}
+	// task lists
+	switch {
+	case bytes.HasPrefix(text, []byte("[ ] ")):
+		text = append([]byte(`<input type="checkbox" disabled="">`), text[3:]...)
+	case bytes.HasPrefix(text, []byte("[x] ")) || bytes.HasPrefix(text, []byte("[X] ")):
+		text = append([]byte(`<input type="checkbox" checked="" disabled="">`), text[3:]...)
+	}
+
 	out.WriteString("<li>")
 	out.Write(text)
 	out.WriteString("</li>\n")