Implement ISG (#188)

I saw this feature being asked for by a few people. Since it shouldn't directly interfere with gameplay, unless specifically activated (pickup item while shielding and swinging). I've checked both on patched and non-patched versions of the game to see if this implementation works, and they both had the same expected behaviour (8 slashes with kokiri sword to kill Gibdos). It appears that this implementation should not impact any regular game play.

hooks/rst_isg.hks

@@ -0,0 +1,12 @@
+# Relative branch to skip past sword flags.
+# b #0x00000c
+patch_in_isg:
+  type: patch
+  addr: 0x1D32E0
+  data: 010000EA
+
+# b #0x00000c
+patch_in_isg_2:
+  type: patch
+  addr: 0x1DBEA4
+  data: 010000EA