add mapper that applies to sql columns

[Minnowo]

I was adding Sqlite as an additional database on one of my apps which is already using Postgres. It seems Postgres always returns column names in lowercase, but not Sqlite.

I ended up hitting a case where I had something like this:

SELECT *, xyz as some_lowercase_name
FROM table

And the column names from * would be uppercase, but not the some_lowercase_name column. Which caused a mapping error:

missing destination name VERSION in *database.TblConfig

This PR adds a new option to the mapper struct, that lets you map the sql column names too.

[mrj0]

Looks good, thank you!