refactor(Sheet): create parse method

Move `Frontmatter.Parse` to `Sheet.parse`, and delete the `frontmatter`
package. `Sheet.parse` more accurately describes the parser's behavior.

internal/sheet/parse_test.go

@@ -0,0 +1,95 @@
+package sheet
+
+import (
+	"testing"
+)
+
+// TestHasFrontmatter asserts that markdown is properly parsed when it contains
+// frontmatter
+func TestHasFrontmatter(t *testing.T) {
+
+	// stub our cheatsheet content
+	markdown := `---
+syntax: go
+tags: [ test ]
+---
+To foo the bar: baz`
+
+	// parse the frontmatter
+	fm, text, err := parse(markdown)
+
+	// assert expectations
+	if err != nil {
+		t.Errorf("failed to parse markdown: %v", err)
+	}
+
+	want := "To foo the bar: baz"
+	if text != want {
+		t.Errorf("failed to parse text: want: %s, got: %s", want, text)
+	}
+
+	want = "go"
+	if fm.Syntax != want {
+		t.Errorf("failed to parse syntax: want: %s, got: %s", want, fm.Syntax)
+	}
+
+	want = "test"
+	if fm.Tags[0] != want {
+		t.Errorf("failed to parse tags: want: %s, got: %s", want, fm.Tags[0])
+	}
+	if len(fm.Tags) != 1 {
+		t.Errorf("failed to parse tags: want: len 0, got: len %d", len(fm.Tags))
+	}
+}
+
+// TestHasFrontmatter asserts that markdown is properly parsed when it does not
+// contain frontmatter
+func TestHasNoFrontmatter(t *testing.T) {
+
+	// stub our cheatsheet content
+	markdown := "To foo the bar: baz"
+
+	// parse the frontmatter
+	fm, text, err := parse(markdown)
+
+	// assert expectations
+	if err != nil {
+		t.Errorf("failed to parse markdown: %v", err)
+	}
+
+	if text != markdown {
+		t.Errorf("failed to parse text: want: %s, got: %s", markdown, text)
+	}
+
+	if fm.Syntax != "" {
+		t.Errorf("failed to parse syntax: want: '', got: %s", fm.Syntax)
+	}
+
+	if len(fm.Tags) != 0 {
+		t.Errorf("failed to parse tags: want: len 0, got: len %d", len(fm.Tags))
+	}
+}
+
+// TestHasInvalidFrontmatter asserts that markdown is properly parsed when it
+// contains invalid frontmatter
+func TestHasInvalidFrontmatter(t *testing.T) {
+
+	// stub our cheatsheet content (with invalid frontmatter)
+	markdown := `---
+syntax: go
+tags: [ test ]
+To foo the bar: baz`
+
+	// parse the frontmatter
+	_, text, err := parse(markdown)
+
+	// assert that an error was returned
+	if err == nil {
+		t.Error("failed to error on invalid frontmatter")
+	}
+
+	// assert that the "raw" markdown was returned
+	if text != markdown {
+		t.Errorf("failed to parse text: want: %s, got: %s", markdown, text)
+	}
+}