Make the title image configurable

User Story

As a contributor I want to define a custom title image for my posts, so that it looks distinctively.

Proposal

• Add the option title-image to the Jekyll front matter.
• The contributor can set three different options:
  • default: displays the default title image
  • none: no title image should be displayed
  • <image>: the given image shall be displayed

Example

---
title: Test
title-image: test-title-image.jpg
layout: default
---
# Content