Support multiple custom mutators (#282)

* Make a list of custom mutators using env variable

* Set up multiple custom mutators

* Add destroy custom mutator and changes to load_custom_mutator

* Use array instead of list, make changes to afl-fuzz-one for multiple mutators

* Make change to fuzz-one custom_queue_get to support multiple mutators

* Modify custom python mutator support

* Fix bug

* Fix missing afl->mutator->data

* Revert to list with max count

* Change custom_pre_save hook and code format

* Free custom_mutator struct in the list

* Add testcase for multiple custom mutators

* Resolve merge conflict

test/test-multiple-mutators.c

@@ -0,0 +1,24 @@
+/**
+ * Test-Case for multiple custom mutators in C
+ * Reference:
+ * https://github.com/bruce30262/libprotobuf-mutator_fuzzing_learning/blob/master/4_libprotobuf_aflpp_custom_mutator/vuln.c
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int main(int argc, char ** argv)
+{
+    int a=0;
+    char s[16];
+    memset(s, 0, 16);
+    read(0, s, 0xa0);
+
+    if ( s[17] != '\x00') {
+        abort();
+    }
+
+    return 0;
+}