Introduce Redcarpet Markdown Engine

Add a dependency on the redcarpet gem to parse the project’s imminent Markdown files.

Additionally, configure Middleman to make use of the parser by:

• setting the :markdown_engine setting, along with additional :markdown configuration values from within config.rb
• renaming the source/index.html.erb to source/index.html.md

The previous file extension sequence was significant, as it encoded a pre-processing sequence of ERB to HTML.

The new sequence encodes that the source files will be written in Markdown (as denoted by the .md), then transformed into HTML (as denoted by the .html).

To ensure that the templating engine change was successful, change the test’s assert_text verification to a more strict assert_selector that asserts that the git-read text appears within an <h1> element, which is what Markdown transforms text underlined with === into.