Added options to fine-tune IV-18 driver

There are now three multiplexing algorithms (digit multiplexing,
subdigit multiplexing, and segment multiplexing) to choose from
at compile-time; see the comments in config.h for details.

With the IV-18 to-spec hack, display brightness may now be controlled
with boost voltage, pulse width modulation, or both simultaneously;
see config.h for details.

Finally, this commit includes a workaround to reduce/eliminate display
flicker during queries to the DS18B20 temperature sensor for clocks
with software temperature compensated timekeeping.