germanic notation on vgm2txt, and txt2vgm is also needed

one thing is needed for vgm2txt is to log tone frequencies in germanic notation as well - i was trying to develop some converters, like vgm2vortextracker (ay-3-8910 and sn76489), and such information is extremely useful for debugging (anyway, i know that most sound chips uses 2 bytes for each channel, so this information would have the current frequency in each moment that the respective frequency register were accessed)

another useful thing is, not only having vgm2txt document being easy to convert back to vgm format, as a reverse tool like txt2vgm would be very welcome - this would help a lot optimizing vgm files by hand on text editors, besides the existing optimization tools

thanks in advance! :)