Automatically infer colorable_fields on create_index

[bmschmidt]

Asking people to pass colorable_fields is a little annoying; if not passed, we could infer colorability based on the following rules.

1. If it's a float or an int, include it.
2. If it's a date, include it.
3. If it's a string, check the cardinality; include as a likely categorical if it has lots of repeats. (note--this is the tricky one, because the cardinality of previously uploaded data isn't necessarily handy when a user creates an index.)