Add support for SSD1331

Hi! I adapted the SSD1351 driver for the SSD1331. The existing SSD1351 driver can't be used e.g. with a new init sequence (see issue #27) because the write_register function for SSD1331 requires the D/C line to be kept in command mode throughout the (potentially) multi-byte register write sequence. There's also not an "enter write mode" command for SSD1331, so D/C needs to be toggled back and forth. This could probably be cleaned up, but works for me.