Using CFLAGS_STD and CPPFLAGS_STD

Hi there!

I tweaked `Arduino.mk` to use `CFLAGS_STD` the flags and added `CPPFLAGS_STD` for `c++`.

They can now be set in your `Makefile`.

Also changed `HISTORY.md` and `arduino-mk-vars.md`.

Hope it helps! :)