Updates to compile-benchmark

[Per Nordlöw]

As a response to the recent speed-ups of DMD, 
https://github.com/nordlow/compiler-benchmark/ has received 
several improvements during the past week. As a result, most 
notably,

- The output has become more readable through compacter table 
layouts and where the output has been sectioned by language into 
3 categories
- Benchmarking is now parallelized
- Language coverage is now broader
- The README.md is now (more) in sync with the code
- A new `--path` flag has been added to the benchmark script for 
better execution control
- Relative metrics is opt-in via `--rel`
- ...

Feel free to try it out and suggest improvements.

Happy benchmarking.