Add `--table` argument + misc bug fixes and improvements

[birdonfire]

• --table argument
• table-related arguments support schema prefixes
• Sequence updating now respects --table and --exclude-table
• Force float division in _completeness_score
• Add timestamp to log output
• Fetch parent rows required by configured constraints
• Respect cross-schema constraints
• Support qualified table names in constraint keys

[catherinedevlin]

Sorry for taking so long to the review this - a combination of busyness and inattention. Lots of work in here, thanks a ton!