libsmartcols: introduce column type

libsmartcols: introduce column type

Now the column can define JSON-type, this commit introduces data-type.
This type is hint for filters and counters how to convert strings from
cells to raw data.

If the conversion from string is not possible, then application can
define callback for this purpose (aka. "datafunc"), this function has
to return the raw data as a void pointer.

This solution allows to use human readable things in cells (e.g.
"123MiB"), but filters and counters can still use the original data.

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