Add postgres importer

[Astn]

This would be a command similar to a file import command. Though instead of importing from a path a particular file type we import from a database using a particular connection string.

Syntax along the lines of:

load postgres "connection string to database"

When a connection is made to the database we introspect the schema to determine what columns are foreign keys.

When mapping keys we should use a format such as

{host}/{database}/{schema}/{table}/{id}

And that mapping has to happen for both primary keys, and foreign keys. Composite keys would be similar, we would delimit each portion of the composite key with a /.

We would want to import the database schema as well. Likely we just use @{table}/{column} in the key. Then we can have a node describing the column.