Command-line processing and Help for 'headers' options.
Implement output headers (without --full).
implement headers support.
key-compare: make 'blanks' public.
bugfix: Allow 'count' to count non-numeric fields
remove unused cruft (from header feature)
tests: test count on non-numeric fields
headers: don't print group headers with --full.
tests: add --header-in tests
bugfix: '--full' and no groups would return the wrong line.

Returned the last line of the last group, instead of the first line of
the last group (as it returns the first line of every other group).
tests: check empty input