Schema (#60): pattern constraint for string types

[mhuang74]

Schema (#60): pattern constraint for string types

Overall, probably only want to generate pattern constraint for columns that have clear structure to their value, but too large to generate enum constraint for. This works in general, but lacks the smarts to recognize common formats, e.g. email address or date timestamp.

• choose columns to generate pattern constraint via --pattern-columns option
• only generate pattern constraint for String types without enum constraint
• updated integration test for schema and validate