WIP: [coverage] Added coverage comparison visualization tool

[harlanhaskins]

Adds a tool, cov-compare, in the /bin directory that either:

• Generates a YAML file representing code coverage for a specific covered build, or
• Compares two historical YAML files and produces either a Markdown summary (suitable for a PR comment) or a detailed HTML report of the difference of coverage between two commits.

[stevepeak]

Hey, I'm the founder of Codecov and would love to extend a warm welcome to Swift to use Codecov. Lets setup a time to chat about this opportunity so I may express the values and company direction with you.

Thank you 👍