lslogins: add JSON output format mode

lslogins: add JSON output format mode

This adds the -J and --json options that enable JSON output mode
for the libsmartcols table. JSON output can be especially useful if
another program wants to consume lslogins's data in an easily parse-
able and language independent format.

Each keys's value will be converted into an appropriate JSON data
type, i.e. integers are numbers, empty values are null and others
are strings.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>