Truncate foreign and reference columns if long.

Anything with more than three keys tends to muck up PrettyTable so I
made the number of keys displayed configurable. If the number of keys in
the column is greater than the allowed, it'll output:
"table1.table1_id, table2.table2_id, (+ 36 more)."
Merge pull request #43 from slackorama/feature/fix-column-width

Truncate foreign and reference columns if long.