don't keep short HTOA's if their peak level is low

Right now morituri will blindly rip PREGAPs before the first track, even if they're silent. This simple patch lets morituri rip the PREGAP, then checks the rip's peak level as reported by cdparanoia to see whether it's below "SILENT" or not (default to 1e-10). If it's silent, it quietly removes the evidence from the log and cue and carries on with the rip, otherwise the original bevahiour is kept.