unsafeSqlExtractSubField to support EXTRACT subfield FROM field-syntax

See issue https://github.com/meteficha/esqueleto/issues/53 . I have successfully tested the following code with PostgreSQL:

```
extractEpoch :: UnsafeSqlFunctionArgument a =>  a -> SqlExpr (Value PersistValue)
extractEpoch = unsafeSqlExtractSubField "EPOCH"
```