Add filtering to sqlite implementation

[grahamrp]

May need to include the row indices to make this work

For column-based operations, e.g. filtering, colnames need to be passed to query, therefore there needs to be a way to share info about the seeding data.frame's column names to the filter function. e.g. a lookup from col-index to col-name

[grahamrp]

The dt filter options are populated from the initial data frame provided, so if this is empty then filter boxes are disabled (because there need to be more than 1 unique value in a column to offer filtering). Ultimately we need a global view of the unique values (or min/max) in each filterable column, and a mechanism for setting these in the dt.