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.go

@@ -0,0 +1,45 @@
+package sheet
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+
+	"gopkg.in/yaml.v1"
+)
+
+// Parse parses cheatsheet frontmatter
+func parse(markdown string) (frontmatter, string, error) {
+
+	// determine the appropriate line-break for the platform
+	linebreak := "\n"
+	if runtime.GOOS == "windows" {
+		linebreak = "\r\n"
+	}
+
+	// specify the frontmatter delimiter
+	delim := fmt.Sprintf("---%s", linebreak)
+
+	// initialize a frontmatter struct
+	var fm frontmatter
+
+	// if the markdown does not contain frontmatter, pass it through unmodified
+	if !strings.HasPrefix(markdown, delim) {
+		return fm, markdown, nil
+	}
+
+	// otherwise, split the frontmatter and cheatsheet text
+	parts := strings.SplitN(markdown, delim, 3)
+
+	// return an error if the frontmatter parses into the wrong number of parts
+	if len(parts) != 3 {
+		return fm, markdown, fmt.Errorf("failed to delimit frontmatter")
+	}
+
+	// return an error if the YAML cannot be unmarshalled
+	if err := yaml.Unmarshal([]byte(parts[1]), &fm); err != nil {
+		return fm, markdown, fmt.Errorf("failed to unmarshal frontmatter: %v", err)
+	}
+
+	return fm, parts[2], nil
+}