Ability to extend upsert expression with options

[atzedus]

This PR adds ability to provide some options to extend UPSERT expression.
For example, following sql needed:

CREATE TABLE test (name text);
CREATE UNIQUE INDEX ON test(lower(name)) WHERE name IS NOT NULL;

INSERT INTO test (name) ON CONFLICT (lower(name)) WHERE name IS NOT NULL DO NOTHING;

Currently it not possible with "Upsert" method.
This PR adds ability to provide options to Upsert method, for example:

m := &models.Test{ name: "test" }

m.Upsert(ctx, db, false, nil, boil.Infer(), boil.Infer(), models.UpsertConflictObject("(lower(name)) WHERE name IS NOT NULL"))

Also, it adds UpsertUpdateSet(string) option to customize DO UPDATE SET expression to any custom value.

[stephenafamo]

This looks quite nice and is fully backwards-compatible. Thank you.