column: make defined separator to be non-greedy

column: make defined separator to be non-greedy

This patch changes interpretation of subsequent delimeter interpretation.
Earlier version merged columns that had null string as content together,
which lead to output as visualized below.

$ printf "a:b:c\n1::3\n" | column  -t -s ':'
a  b  c
1  3

The number 3 has wrong column, which this patch takes care of, and alters
the output following way.

$ printf "a:b:c\n1::3\n" | column  -t -s ':'
a  b  c
1     3

This patch does not alter the default case, e.g., subsequent white spaces
are understood as separator of the same field, and the beginning of line
white spaces are being ignored together.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>