parser: Refactor frontmatter parser and add tests

Lots of cleanups here:

- Refactor InterfaceToConfig and InterfaceToFrontMatter to use io.Writer.
- Simplify InterfaceToFrontMatter by wrapping InterfaceToConfig.
- Export FrontmatterType since we return it in DetectFrontMatter.
- Refactor removeTOMLIdentifier to avoid blindly replacing "+++".
- Update HandleJSONMetaData to return an empty map on nil input.
- Updates vendored goorgeous package and test for org-mode frontmatter.
- Add tests and godoc comments.

Coverage for parser package increased from 45.2% to 85.2%.

parser/page_test.go

@@ -0,0 +1,130 @@
+package parser
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPage(t *testing.T) {
+	cases := []struct {
+		raw string
+
+		content     string
+		frontmatter string
+		renderable  bool
+		metadata    map[string]interface{}
+	}{
+		{
+			testPageLeader + jsonPageFrontMatter + "\n" + testPageTrailer + jsonPageContent,
+			jsonPageContent,
+			jsonPageFrontMatter,
+			true,
+			map[string]interface{}{
+				"title": "JSON Test 1",
+				"social": []interface{}{
+					[]interface{}{"a", "#"},
+					[]interface{}{"b", "#"},
+				},
+			},
+		},
+		{
+			testPageLeader + tomlPageFrontMatter + testPageTrailer + tomlPageContent,
+			tomlPageContent,
+			tomlPageFrontMatter,
+			true,
+			map[string]interface{}{
+				"title": "TOML Test 1",
+				"social": []interface{}{
+					[]interface{}{"a", "#"},
+					[]interface{}{"b", "#"},
+				},
+			},
+		},
+		{
+			testPageLeader + yamlPageFrontMatter + testPageTrailer + yamlPageContent,
+			yamlPageContent,
+			yamlPageFrontMatter,
+			true,
+			map[string]interface{}{
+				"title": "YAML Test 1",
+				"social": []interface{}{
+					[]interface{}{"a", "#"},
+					[]interface{}{"b", "#"},
+				},
+			},
+		},
+		{
+			testPageLeader + orgPageFrontMatter + orgPageContent,
+			orgPageContent,
+			orgPageFrontMatter,
+			true,
+			map[string]interface{}{
+				"TITLE":      "Org Test 1",
+				"categories": []string{"a", "b"},
+			},
+		},
+	}
+
+	for i, c := range cases {
+		p := pageMust(ReadFrom(strings.NewReader(c.raw)))
+		meta, err := p.Metadata()
+
+		mesg := fmt.Sprintf("[%d]", i)
+
+		require.Nil(t, err, mesg)
+		assert.Equal(t, c.content, string(p.Content()), mesg+" content")
+		assert.Equal(t, c.frontmatter, string(p.FrontMatter()), mesg+" frontmatter")
+		assert.Equal(t, c.renderable, p.IsRenderable(), mesg+" renderable")
+		assert.Equal(t, c.metadata, meta, mesg+" metadata")
+	}
+}
+
+var (
+	testWhitespace  = "\t\t\n\n"
+	testPageLeader  = "\ufeff" + testWhitespace + "<!--[metadata]>\n"
+	testPageTrailer = "\n<![end-metadata]-->\n"
+
+	jsonPageContent     = "# JSON Test\n"
+	jsonPageFrontMatter = `{
+	"title": "JSON Test 1",
+	"social": [
+		["a", "#"],
+		["b", "#"]
+	]
+}`
+
+	tomlPageContent     = "# TOML Test\n"
+	tomlPageFrontMatter = `+++
+title = "TOML Test 1"
+social = [
+	["a", "#"],
+	["b", "#"],
+]
++++
+`
+
+	yamlPageContent     = "# YAML Test\n"
+	yamlPageFrontMatter = `---
+title: YAML Test 1
+social:
+  - - "a"
+    - "#"
+  - - "b"
+    - "#"
+---
+`
+
+	orgPageContent     = "* Org Test\n"
+	orgPageFrontMatter = `#+TITLE: Org Test 1
+#+categories: a b
+`
+
+	pageHTMLComment = `<!--
+	This is a sample comment.
+-->
+`
+)