Add code coverage badge

[tylermilner]

Investigate and integrate the code coverage badge into the Readme. Codecov looks to be one possible provider, but there could be others.

[achappell]

I've used codecov and really like it.

[achappell]

Code beat is another cool repo health tool that could be cool to look at

[tylermilner]

At first glance, Codebeat looks awesome! I couldn't tell if it does the code coverage badge, but even if it doesn't, we should look into integrating with it. It looks pretty awesome for internal projects too.

[achappell]

It didn't do code coverage, we would want badges for both. Here is an example of one of my projects that uses it. It does some linting, but also find duplication and complexity issues. https://codebeat.co/projects/github-com-achappell-dungeonsanddragonscharactersheet

[wmcginty]

+1 for Codebeat. I've used it a few times before and it's very useful.