Calculate and report simple signature

[obo]

We should come up with a simple signature that people can copy-paste. The signature should include:

• elitr-testset commit ID (just a few digits, the short git commit ID, not the full one)
• possibly SLTev commit ID (again, just the short one)
  • I am not really sure
• then the calculation flags (casing, mwerSegmenter, ...)
• and then metric name

Probably the detailed output should be as it is now, with "signature prefix" listed above the table. Something like:

Signature: SLTev_e3da41-elitrtestset_4a5d99
nnPC   WER    ...
...

The --simple output (see #8) should probably already concatenate it and show things like:

SLTev_e3da41-elitrtestset_4a5d99-nnPC-WER  ...
...

[mohammad2928]

It's done.