Option to remove unnecessary white lines from output. 

Because of Node.js spawn log output, every other line in the output is a blank line, which doesn't look good, specially when using test runner like mocha This provides an way to correct that. This option should probably be escalated to Grunt.util.spawn.

For instance this:

![screenshot 2013-12-27 13 02 49](https://f.cloud.github.com/assets/418288/1813285/32b19e66-6eb4-11e3-8d84-c4e6790ca5a9.png)

will become:
![screenshot 2013-12-27 13 00 03](https://f.cloud.github.com/assets/418288/1813281/dbb1607e-6eb3-11e3-8d1f-f0e4c9603f93.png)