Use Asyncio to execute

[mathben]

To increase speed of execution, I suggest to implement asyncio for each writer when command summary.

https://github.com/roskakori/pygount/blob/fac7f7beb760f3edca1d202e684ff4d401f2c462/pygount/command.py#L322-L349

What do you think?

[roskakori]

Probably makes sense although the error reporting might need some changes with possibly multiple readers/writers failing at the same time.

Not high on my priority list though, but I leave it in the backlog.