Adding the Gemfiles for bundler to set up the environment, and fixed a minor bug in the _config.yml

The changes include:

1. Adding Gemfile, and Gemfile.lock. This would allow a user to quickly set up their development environment with jekyll and other dependencies if it is not already set up on the system. I've also added the gem, wdm, that is needed in the windows environment for a better development experience with jekyll.

2. Replaced the deprecated key, pygments, in the _config.yml with the key - highlighter, and it's set to pygments.