[LUA 5.1] io.open

This function opens a file, in the mode specified in the string mode.
It returns a new file handle, or, in case of errors, nil plus an error message.

The mode string can be any of the following:

• "r": read mode (the default);
• "w": write mode;
• "a": append mode;
• "r+": update mode, all previous data is preserved;
• "w+": update mode, all previous data is erased;
• "a+": append update mode, previous data is preserved,
    writing is only allowed at the end of file.

The mode string may also have a 'b' at the end, which is needed in some systems to open the file in binary mode.
This string is exactly what is used in the standard C function fopen.