Add helper function for retrieving libmutton version

0.go

@@ -95,3 +95,5 @@ func safeBoolDeref(b *bool) bool {
 	}
 	return *b
 }
+
+func main() {}

README.md

@@ -98,6 +98,7 @@ int main() {
     - [X] ~~RCWDArgument()~~ (RCWD not supported)
 - [X] global
     - [X] ~~VAR (CB func): GetPassword~~ (RCWD not supported)
+    - [X] VAR LibmuttonVersion (GetVersion())
     - [X] DirInit(preserveOldCfgDir bool) (string, error)
     - [X] ~~GenDeviceIDList() ([]fs.DirEntry, error)~~ (not for use outside of libmutton)
     - [X] GetCurrentDeviceID() (string, error)

global.go

@@ -65,4 +65,10 @@ func GetVanityPath(realPath *C.char) C.PascalString {
 	return getPascalString(global.GetVanityPath(C.GoString(realPath)))
 }
 
-func main() {}
+// GetVersion returns:
+// versionNumber
+//
+//export GetVersion
+func GetVersion() *C.char {
+	return C.CString(global.LibmuttonVersion)
+}