Only initialize one instance of flake8.

In my project, we have a full backend application (with tests, totaling 581 files), each spawning an instance of flake8 for each file.

[Code for this strategy is here](https://github.com/btholt/grunt-flake8/commit/825a996c7f6eab153cae325d8f861116fef70ec2#diff-7bc7c3ebac5dfac782a7827b2172d1ffR115).

Running the `grunt flake8` task, using this strategy, takes approximately 15 seconds.

By running one instance of the flake8 command, it is reduced to about 2.5 seconds.

The grunt task should be switched to a synchronous, blocking call of one flake8 process, which will be responsible for linting the entire project.