Read from files concurrently

[mvdan]

As suggested by @bep, we could read and run the regex over files passed as arguments concurrently all at once, instead of one after the other. For regular files this doesn't make much sense in the general case, but it could make sense in files that cause blocking reads like named pipes or stuff that goes over the network.

The only downside I can see to this is that it's a bit overkill for the generic, simple case.

[mvdan]

I'm not going to implement this. For edge cases where you want it to be asynchronous among files, you can fire up multiple processes. This way we don't overcomplicate the generic use case.