Allow to escape nil in FileDestination

[thbar]

This is a follow-up to this code and this comment.

MySQL bulk load will apparently convert \N into null values if passed.

It would be nice to add a configurable support for that.

Acceptance

• this code should have tests
• should be on a separate feature branch
• unlike the original code, no transforming to UTF-8 (this must be done elsewhere, people are dumping non UTF-8 data as well!)
• if a parameter named escape_nil_as is passed, use its value to replace nils by this value
• if the parameter is not passed, no escaping should occur.

I did a bit of research and \N seems to work as well for PostgresSQL.

What do you think guys? (especially @sgrgic and @josephbridgwaterrowe)