Add option to specify frida/-server path on device

[sn0opy]

Because of issue #71 I was looking into placing my frida-server into /data/local/tmp which does work if using it manually via frida, but not if I want to use Dwarf.

I think it would make more sense to use the absolute path, since Dwarf wants to install it into /system/bin|xbin anyway, thus adding a config option to specify the path myself while having the /system path as a fallback would be great.