Support PostgreSQL connections for source data

This allows for an OGR-compatible PostgreSQL connection string, such as
"PG:dbname=pdx_bldgs user=emma host=localhost" to be given in place of a
source file path.

It also adds a --sql option to query the data prior to export. This is
particularly useful for exporting a single table from a complex database,
via something like “SELECT * FROM buildings”

Closes #20
can a SQL source have multiple layers?