Allow .hh as header file extension

[mikaelpatel]

The Arduino library handling and build requires .h as extension of header files. It would be great to also allow .hh as extension as this is often used for C++ and is an important trigger for C++ mode in GNU Emacs.

Best regards, Mikael

[matthijskooijman]

Sounds good to me. Care to submit a PR (against 1.5.x is probably best)?