Adding merging functionality

[nine-inch-rails]

With regards to this: #34

I've made it so that you can pass a flag to start generating a report and merge it with the existing json (rspec only, though extending the functionality shouldn't be so tough).
The flag is: KNAPSACK_MERGE_REPORT=true and is set with the KNAPSACK_GENERATE_REPORT flag

This addition does two things:
If a new spec(s) is added, its/their runtime is added to the existing json
If it's run against the suite, the existing times will be overwritten with the new ones via hash merge, 'updating' them

[wscourge]

@ArturT @shadre is this going to get merged? Do you guys still maintain this repo, or should I fork and use my own?

[ArturT]

@wscourge At this moment, we abstain from the decision to introduce new features.

I hope in 2023, we will clarify the high-level plans so that we can put more focus on delivering value to users. We will better understand how the knapsack gem fits into that.

[wscourge]

OK, looking forward to it - thanks!