mapper: moved mapper to separate file, added snake case mapper

Performance is comparable to sting.ToLower

BenchmarkSnakeCase-4              200000              5922 ns/op            1216 B/op         97 allocs/op
BenchmarkToLower-4                300000              5418 ns/op             704 B/op         74 allocs/op

gocqlx.go

@@ -4,18 +4,11 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
-	"strings"
 
 	"github.com/gocql/gocql"
 	"github.com/jmoiron/sqlx/reflectx"
 )
 
-// DefaultMapper uses `db` tag and strings.ToLower to lowercase struct field
-// names.  It can be set to whatever you want, but it is encouraged to be set
-// before gocqlx is used as name-to-field mappings are cached after first
-// use on a type.
-var DefaultMapper = reflectx.NewMapperFunc("db", strings.ToLower)
-
 // structOnlyError returns an error appropriate for type when a non-scannable
 // struct is expected but something else is given
 func structOnlyError(t reflect.Type) error {