Convert Markdown to be species independent

[schae234]

Right now the website is horse specific, but there are many species that would benefit from manual gene annotation. Make the wesbite species agnostic and code the project name and the species name as Jekyll variables.

Jekyll will take variables from _config.yml and convert them into the appropriate text when it translates markdown to HTML. An example of this is currently in contributing.md where the tag, {{site.title}}, is converted to the appropriate name in the HTML text (rendered by Jekyll).

• Convert all instances of HorseGeneAnnotation to the {{site.title}} tag.