Support PWM pins of ATtiny85 Timer1

[jfpoilpret]

Currently, FastArduino ATtiny85 support enables only 2 PWM pins, both based on Timer0.

Event though currently FastArduino does not allow support for PWM pins linked to more than 1 timer, that still leaves room for pin PB4 (OC1B).

[jfpoilpret]

The reason why PB4 is not initially supported by FastArduino ATtiny85, is because in ATtiny85, Timer1 is very different to all other AVR timers, and support would have required FastArduino refactoring of "board traits" in addition to Timer implementation.

I'd rather implement this refactoring for a later FastArduino version.

[jfpoilpret]

Resolving this issue would better be combined with #18

[jfpoilpret]

Note the following article may be useful for this issue: https://www.re-innovation.co.uk/docs/fast-pwm-on-attiny85/

[jfpoilpret]

Remove this issue from milestone 1.3
To be rescheduled later.
This does not seem urgent (not so many projects use ATtinyX5 MCU).