Add -p flag to disable writing of cache and temporary files

Closes #285.

thumbs.c

@@ -25,6 +25,7 @@
 #include <unistd.h>
 #include <utime.h>
 
+#include "options.h"
 #include "thumbs.h"
 #include "util.h"
 
@@ -83,6 +84,9 @@ void tns_cache_write(Imlib_Image im, const char *filepath, bool force)
 	struct utimbuf times;
 	Imlib_Load_Error err = 0;
 
+	if (options->private_mode)
+		return;
+
 	if (stat(filepath, &fstats) < 0)
 		return;
 
@@ -270,7 +274,7 @@ bool tns_load(tns_t *tns, int n, bool force, bool cache_only)
 				cache_hit = true;
 			}
 #if HAVE_LIBEXIF
-		} else if (!force) {
+		} else if (!force && !options->private_mode) {
 			int pw = 0, ph = 0, w, h, x = 0, y = 0;
 			bool err;
 			float zw, zh;