Add support for thresholds

[larsthorup]

I wanted to have the ability to fail the build when a coverage threshold is not met. This is already possible with Istanbul. This pull request adds this feature when using Istanbul with Grunt. Let me know what you think!

[larsthorup]

Thanks! I added thresholds of 100 to Gruntfile.js, which will not produce any warning. However if you manually change them to, say, 200, they will produce warnings. Don't know how we can verify the warning scenario automatically though, let me know if you have a suggestion. Maybe as simple as a seperate target: 'grunt test:shouldfail'?

[maenu]

Hm, good point. When I think about it, testing that a warning is produced really not as simple as I thought, since subsequent tasks after a warning are only executed when forced. So the same pattern as used in the already existing integration test can not be applied. Maybe mocking grunt.warn() to set a global flag and check this one afterwards may be possible.

[larsthorup]

I've added a test using your suggestion, see my additional commit. Thanks!

[larsthorup]

Let me know if there is more I can do to facilitate the acceptance of this pull request. Thanks!

[maenu]

I just couldn't find any time for this project in the last two weeks. But I have the next few days off and this is on the top of my to-do list.

[larsthorup]

Excellent. Thank you!