Add optional command arguments in the end for "attach" command

[DMaroo]

This PR adds extra arguments at the end of "attach" command, and these will be forwarded on to perf verbatim. Therefore, we can now use the following command

$ magic-trace attach -p $(pidof foo) -trigger bar sleep 5

The perf command called under the hood would be

perf <args> sleep 5

Done in collaboration with @Mantra-7.

Fixes #230.