[WIP] rtl_fm: add initial -E split prototype

[rxseger]

https://github.com/rxseger/rx_tools/issues/31 rx_fm: split audio files per squelch

• [x] Open new files per squelch
• [ ] Frequency in filename
• [ ] Custom filename format string
• [ ] generate_header() on each if specified (-E wav)
• [ ] Buffer and only save files if beyond a certain size