Add knapsack binary

[strika]

Artur, thanks for the great work with Knapsack.

I'm interested if you're open to including the knapsack executable? Let me explain my motivation. I did some experimentation with Knapsack where I didn't use Knapsack to generate RSpec report. Instead, I generated the report from some other reports that I collect about specs. Then I used the manually built Knapsack report to run specs in parallel.

In this setup, Knapsack doesn't need to be included in the Gemfile. Instead, I can install Knapsack globally on the build machine and reuse it for all projects that I have. Do you think this can be interesting to a wider audience?

The PR implementation might not be the best, but it illustrates the idea. I can install gem globally and execute RSpec with knapsack rspec.

What do you think about this? Thanks.

[ArturT]

@strika Thanks for PR. I will add case for cucumber, minitest and update read me then release new gem version.