kiji-modeling: EXP-275. Make the missing column values option for KijiInput more extensible.

issue: https://jira.kiji.org/browse/EXP-275
review: https://review.kiji.org/r/1340
commit: https://github.com/kijiproject/kiji-modeling/commit/8561b49ba362820980f526c3c8db1ee0ae66091e