Possibility to specify compilation flags to arduino-builder from the IDE

[clasqui]

Description

Add a space on the Arduino IDE to specify compilation settings, specifically flags to be passed to arduino-builder and the avr toolchain, like definitions.

Justification

It would be very useful to be able to define some constants at compile time, for example when there is a secret value that you don't want to put in your code (maybe because you open source your project). Therefore, you could tell the IDE to use the -D flag with your value when compiling, and then the preprocessor would add that value to the program.

[per1234]

Closing as duplicate of #421