Display row counts, table size, etc in output messages

[chdsbd]

Related to #116

We could accept an input file that would include table name, row count, index size, table size, etc. and use that information to display more useful error messages.

If a migration is going to lock a table while it scans the whole table, that doesn't matter for a 300 row table, but does for a 3,000,000 row table.

[jelder]

Are rules that require access to a database welcome in squawk?

That said, the database instance which migrations are developed against is usually not biggest database that it will be run against. And locking a million-row table that gets no queries may be ok, but locking a thousand-row table that gets queried constantly could be bad.

Printing warnings based on the size of tables in a development environment may be misleading.

[chdsbd]

@jelder We don't have any rules that connect to the database, but I'm definitely open to ideas for rules that require more database information

I think my original idea for this issue was that you'd run some SQL on your production database, then Squawk would use that information for linting

[jelder]

@chdsbd One thing I'd like to have is a warning that a foreign key constraint isn't indexed.

Edit: Moved to #334