Update <title> tag inside header.html in Minimal-Mistakes Gem file

[robinkwilson]

1. Copy _includes/header.html from Minimal-Mistakes Gem and paste into _includes/header.html in project root
2. Update relative references inside header.html to reference the location of Minimal-Mistakes url.
3. Reference site.title, then if page.title exists add ": " + page.title to the <title></title> content.