Add --log-append option

Add --log-append option which makes tio append to any existing log file.

This also changes the default behaviour of tio from appending to
overwriting any existing log file. Now you have to use this new option
to make tio append.

src/options.h

@@ -48,6 +48,7 @@ struct option_t
     unsigned int ri_pulse_duration;
     bool no_autoconnect;
     bool log;
+    bool log_append;
     bool log_strip;
     bool local_echo;
     enum timestamp_t timestamp;