Fix the repo name

I accidentally named the repo 'go-shellutil' instead of 'go-shellquote'.
Rename the repo and update the README.
diff --git a/README b/README
index 34af739..87bb00d 100644
--- a/README
+++ b/README
@@ -1,7 +1,7 @@
 PACKAGE
 
 package shellquote
-    import "github.com/kballard/go-shellutil"
+    import "github.com/kballard/go-shellquote"
 
     Shellquote provides utilities for joining/splitting strings using sh's
     word-splitting rules.