feat: define xorm.FromDialectAndDB #1812

Hi! I use your greate library, xorm.

I'd like to use xorm.Engine with my own *sql.DB instances. Because I also want to use SessionResetter interface, so I want to call sql.OpenDB to create my own *sql.DB instance before creating *xorm.Engine instance.

So I define FromDialectAndDB function in package xorm.

Thanks!

 // If you do not want to use your own dialect or db, please use NewEngine.
 // For creating dialect, you can call dialects.OpenDialect. And, for creating db,
 // you can call core.Open or core.FromDB.
 func FromDialectAndDB(driverName, dataSourceName string, dialect dialects.Dialect, db *core.DB) (*Engine, error) {