v1.2.0 to fix multiple-definitions linker error

### Releases v1.2.0

1. Fix `multiple-definitions` linker error. Drop `src_cpp` and `src_h` directories
2. Add feature to select among highest, medium or lowest accuracy for Timers for shortest, medium or longest time
3. Fix reattachInterrupt() bug. Check [bugfix: reattachInterrupt() pass wrong frequency value to setFrequency() #19](khoih-prog/ESP8266TimerInterrupt#19)
4. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project
5. Improve accuracy by using `double`, instead of `uint32_t` for `dutycycle`, `period`. Check [Change Duty Cycle khoih-prog#1](khoih-prog#1 (comment))
6. Update examples accordingly