Add to `open` the missing modes that PHP `fopen` has

In Python open (Python docs: open), we have the following modes available: r - reading only, file must exist. w - writing only, file will be created if it does not exist, file will be truncated if it does exist. x - writing only, file must not exist, and will be created. a - writing only, file will be created if it does not exist, all writes write at end of file. r+ - reading and writing, file must exist. w+ - reading and writing, file will be created if it does not exist, file will be tru...