datamash: new option '--narm': ignore NA/NaN values

Suggested by Brandon Invergo:
http://lists.gnu.org/archive/html/bug-datamash/2014-11/msg00002.html

* src/utils.{h,c}: is_na() new function
* src/field-ops.{c,h}:
    field_op_collect(): detect and skip NA/NaN values.
    field_op_summarize_empty(): print results of empty values.
    field_op_summarize(): call above function if no values.
* src/datamash.c: update usage(), main()
* doc/datamash.texi: mention new option
* tests/datamash-tests-2.pl: add tests for new option.