chore: removes the tj/front dependency

Removes the `tj/front` dependency due to licensing concerns. See #490.

vendor/github.com/tj/front/Readme.md

@@ -1,16 +0,0 @@
-# Front
-
-Frontmatter unmarshaller, couldn't find one without a weird API.
-
-## Badges
-
-[![GoDoc](https://godoc.org/github.com/tj/front?status.svg)](https://godoc.org/github.com/tj/front)
-![](https://img.shields.io/badge/license-MIT-blue.svg)
-![](https://img.shields.io/badge/status-stable-green.svg)
-[![](http://apex.sh/images/badge.svg)](https://apex.sh/ping/)
-
----
-
-> [tjholowaychuk.com](http://tjholowaychuk.com)  · 
-> GitHub [@tj](https://github.com/tj)  · 
-> Twitter [@tjholowaychuk](https://twitter.com/tjholowaychuk)

vendor/github.com/tj/front/front.go

@@ -1,24 +0,0 @@
-// Package front provides YAML frontmatter unmarshalling.
-package front
-
-import (
-	"bytes"
-
-	"gopkg.in/yaml.v1"
-)
-
-// Delimiter.
-var delim = []byte("---")
-
-// Unmarshal parses YAML frontmatter and returns the content. When no
-// frontmatter delimiters are present the original content is returned.
-func Unmarshal(b []byte, v interface{}) (content []byte, err error) {
-	if !bytes.HasPrefix(b, delim) {
-		return b, nil
-	}
-
-	parts := bytes.SplitN(b, delim, 3)
-	content = parts[2]
-	err = yaml.Unmarshal(parts[1], v)
-	return
-}