Add result descriptions in mutmut browse

[Otto-AA]

Add result descriptions in mutmut browse.

• add basic description for all results
• add exit codes for all results (maybe helps with debugging / unexpected statuses)
• add expected + actual time for timeouted mutants
• mention which test case killed a mutant
• show error for killed mutants (maybe only in mutmut show?)
• somehow mention all relevant test cases, with some limit / scrolling / ...

[Otto-AA]

I've added a new action "t" to view the tests for the selected mutant.

On a small screen, we will have a scroll option for the diff:

[Otto-AA]

If that looks good to you, I'd merge it as it is @boxed .

I'll postpone the support for "which test killed this mutant" / "what was the exception that killed this mutant". This will require inter-process communication between the parent process and the forked process (Pipe, Queue, etc.). Currently, the info we get from the forked process is limited to exit code and duration.

[boxed]

I like it.