Support .cc extension

[rosshadden]

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Configuration

Operating system: Arch Linux

PlatformIO Version (platformio --version): 3.3.0

Description of problem

platformio does not seem to work if I name my file main.cc, which is the common convention on Unix systems (even though the difference is purely historical and both are widely accepted). Making the extension .cpp works fine. Obviously not a big deal, but I think it should support .cc.