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/log.c

@@ -69,8 +69,17 @@ int log_open(const char *filename)
 
     log_filename = filename;
 
-    // Open log file in append write mode
-    fp = fopen(filename, "a+");
+    // Open log file
+    if (option.log_append)
+    {
+        // Appends to existing log file
+        fp = fopen(filename, "a+");
+    }
+    else
+    {
+        // Truncates existing log file
+        fp = fopen(filename, "w+");
+    }
     if (fp == NULL)
     {
         tio_warning_printf("Could not open log file %s (%s)", filename, strerror(errno));