Add persistent configuration

Save mode and color any time mode changes, or if in MODE_SOLID and
color changes.  Configuration is saved in internal EEPROM.  The
PIC16LF1933 claims 100K writes.  At 5 hrs per day, thats over 13
years of config writes assuming MODE_SOLID and a color change 4
times per year.

When the configuration changes, a 3 second timer is set.  When the
timer expires, then the configuration is written.  This should help
reduce reduce unnecessary EEPROM writes when the uesr is cycling
through the modes to find the one they want.