Unable to use more than one source file in a program

[MrPointer]

The #25 PR unveiled a critical bug where an Arduino program created with add_arduino_executable can't have more than one source files.
The reason for this is that the function attempted to be very similar to CMake's add_executable, however, the CMake function is actually an internal function written in C++, while our function is pure CMake.
It creates a situation where variable arguments in a function can't be "just passed", but instead need to be parsed, preferably by CMake's internal cmake_parse_arguments function.

A PR with a fix should be available soon.