libsmartcols: implement filter based counters

libsmartcols: implement filter based counters

* add support for empty filter; the result of the empty expression is
  always "true"

* Filter can hold arbitrary number of counters

* A counter is based on a built-in function. Now supported are count, max, min and sum functions.

* The default function is "count" (count lines)

* It's possible to assign column (param) to the counter, for example count sum
  of all data in the column

* The param is used in the same way as params in the filter (and
  initialized in the same way)

Based on Masatake YAMATO's work on lsfd.

Signed-off-by: Karel Zak <kzak@redhat.com>