Add option to change signals

[Bojun-Seo]

Implementation of issue #22

heaptrace uses SIGUSR1 and SIGUSR2 to dump allocation status (in terms of size and count respectively) during process running. But the signal handler will not work properly, if the target process uses one or both of the signals.

So I suggest to add option to change signals to dump allocation status. For example,

# register signal 33 to dump the allocation status sorted in terms of size
$ heaptrace --signal size:33 ./a.out

# register signal 34 to dump the allocation status sorted in terms of count
$ heaptrace --signal count:34 ./a.out

# register signal 33 and 34 to dump the allocation status sorted in terms of size and count respectively
$ heaptrace --signal size:33,count:34 ./a.out