Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

markup/highlight/highlight_test.go

@@ -0,0 +1,87 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package highlight provides code highlighting.
+package highlight
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestHighlight(t *testing.T) {
+	c := qt.New(t)
+
+	lines := `LINE1
+LINE2
+LINE3
+LINE4
+LINE5
+`
+
+	c.Run("Basic", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		h := New(cfg)
+
+		result, _ := h.Highlight(`echo "Hugo Rocks!"`, "bash", "")
+		c.Assert(result, qt.Equals, `<div class="highlight"><pre class="chroma"><code class="language-bash" data-lang="bash"><span class="nb">echo</span> <span class="s2">&#34;Hugo Rocks!&#34;</span></code></pre></div>`)
+		result, _ = h.Highlight(`echo "Hugo Rocks!"`, "unknown", "")
+		c.Assert(result, qt.Equals, `<pre><code class="language-unknown" data-lang="unknown">echo "Hugo Rocks!"</code></pre>`)
+
+	})
+
+	c.Run("Highlight lines, default config", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		h := New(cfg)
+
+		result, _ := h.Highlight(lines, "bash", "linenos=table,hl_lines=2 4-5,linenostart=3")
+		c.Assert(result, qt.Contains, "<div class=\"highlight\"><div class=\"chroma\">\n<table class=\"lntable\"><tr><td class=\"lntd\">\n<pre class=\"chroma\"><code><span class")
+		c.Assert(result, qt.Contains, "<span class=\"hl\"><span class=\"lnt\">4")
+
+		result, _ = h.Highlight(lines, "bash", "linenos=inline,hl_lines=2")
+		c.Assert(result, qt.Contains, "<span class=\"ln\">2</span>LINE2\n</span>")
+		c.Assert(result, qt.Not(qt.Contains), "<table")
+
+		result, _ = h.Highlight(lines, "bash", "linenos=true,hl_lines=2")
+		c.Assert(result, qt.Contains, "<table")
+		c.Assert(result, qt.Contains, "<span class=\"hl\"><span class=\"lnt\">2\n</span>")
+	})
+
+	c.Run("Highlight lines, linenumbers default on", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		cfg.LineNos = true
+		h := New(cfg)
+
+		result, _ := h.Highlight(lines, "bash", "")
+		c.Assert(result, qt.Contains, "<span class=\"lnt\">2\n</span>")
+		result, _ = h.Highlight(lines, "bash", "linenos=false,hl_lines=2")
+		c.Assert(result, qt.Not(qt.Contains), "class=\"lnt\"")
+	})
+
+	c.Run("Highlight lines, linenumbers default on, linenumbers in table default off", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		cfg.LineNos = true
+		cfg.LineNumbersInTable = false
+		h := New(cfg)
+
+		result, _ := h.Highlight(lines, "bash", "")
+		c.Assert(result, qt.Contains, "<span class=\"ln\">2</span>LINE2\n<")
+		result, _ = h.Highlight(lines, "bash", "linenos=table")
+		c.Assert(result, qt.Contains, "<span class=\"lnt\">1\n</span>")
+	})
+}