add the settable sqlx.NameMapper which is a 'func(s string) string' and is run on field names;  it defaults to strings.ToLower.  i'm opting out of the more flexible FieldMapper in pull request 8 / issue #6 because I don't want sqlx *usage* to look too different;  I want all sqlx code to obey the 'db' tag, for instance, and not to require other tags because of customizations