Request for a field-separator argument

[peterjanes]

Per #14, "if someone later wants a field-separator argument, make an issue for that."

My particular use case is dealing with large NDJSON streams, where it would be useful to be able to set a field-separator of ,. (There might be cases where a , is found within a key or value, but I don't know if multi-character delimiters is a legit request.)

[timbray]

I think it'd be a dead easy PR, use Regexp.Split, and then some changes to config.c and runner.c for the new argument. But, the performance is going to take a pretty severe hit compared to the brute-force state machine in keyfinder.go - that should be measured and documented.

[timbray]

Addressed in #23