Add code coverage to build

[relentless]

It should be possible to add coverage to the build using something like OpenCover: http://fsharp.github.io/FAKE/apidocs/fake-opencoverhelper.html

I'm not sure what the best way to surface the results is, but just logging the overall coverage tin the FAKE script should do the job.

[relentless]

There are now scripts in the CodeCoverage folder to calculate coverage and create an HTML report manually, still needs adding into build though.

[haf]

see https://github.com/haf/expecto/issues/219#issuecomment-374810994

[TheAngryByrd]

I'll have a better example soon using the msbuild tasks from AltCover.

[TheAngryByrd]

Here's what I have setup for code coverage with AltCover and Expecto. I do use the Expecto vs test runner to make msbuild play nice. https://github.com/TheAngryByrd/MiniScaffold/blob/test-coverage/Content/tests/Directory.Build.targets