Adds optional support for database host and port

[mholthausen]

Description

The following changes are applied with this PR:

• extends the export from a database with the optional specification of a host and a port which is used by psycopg2.connect()
• modification of the documentation with regard to the proposed changes in the export module

Type of change: FEATURE

[delhomer]

Thanks for your contribution @mholthausen, and thanks for considering the documentation! That sounds good to me.

More generally I'm wondering if we should prefer using a configuration file in order to manage database connections. This is quite out of scope there I will open a related issue; I'm OK for merging this one.

[mholthausen]

Thanks, adjusted the documentation so there are no merge conflicts at this time and it may be ready to merge.

[autra]

thanks @mholthausen!