Add insert ignore support for postgres and sqlite

[zhongr3n]

Summary

Add insert ignore support for postgres and sqlite.

For pg, this is ON CONFLICT DO NOTHING, for sqlite it's INSERT OR IGNORE

• https://www.postgresql.org/docs/current/sql-insert.html
• https://www.sqlite.org/lang_insert.html

Testing

• Added new unit tests
• Tested against local postgres server using this library (changes in another private repo)
• Manually tested sqlite query on https://sqliteonline.com/

[coveralls]

Coverage decreased (-0.1%) to 96.063% when pulling fc1e041 on zhongr3n:zhongr3n/fix-insert-ignore into de26a75 on huandu:master.

[huandu]

LGTM. Thanks for your contribution!