Node.js module to control avconv

Usage:

    avconv.call(
        [ ... ],                // array of avconv flags
        callback                // function to call after avconv completed
    );

    avconv.webm(
        input,                  // input source
        callback                // function to call after avconv completed
    );

Requirements:

    Libav avconv compiled with libvorbis, libtheora, x264, libvpx and
    libvo_aacenc.