Add plugin.cpp example to contrib directory

Additionally adds VERSION file to .gitignore

.gitignore

@@ -1,5 +1,6 @@
 # Miscellaneous stuff
 
+VERSION
 .DS_Store
 .sass-cache
 *.gem

contrib/plugin.cpp

@@ -0,0 +1,30 @@
+#include <cstring>
+#include <iostream>
+#include <stdint.h>
+#include "sass_values.h"
+
+// gcc: g++ -shared plugin.cpp -o plugin.so -fPIC -Llib -lsass
+// mingw: g++ -shared plugin.cpp -o plugin.dll -Llib -lsass
+
+union Sass_Value* call_fn_foo(const union Sass_Value* s_args, void* cookie)
+{
+  // we actually abuse the void* to store an "int"
+  return sass_make_number((intptr_t)cookie, "px");
+}
+
+extern "C" const char* ADDCALL libsass_get_version() {
+  return libsass_version();
+}
+
+extern "C" Sass_C_Function_List ADDCALL libsass_load_functions()
+{
+  // allocate a custom function caller
+  Sass_C_Function_Callback fn_foo =
+    sass_make_function("foo()", call_fn_foo, (void*)42);
+  // create list of all custom functions
+  Sass_C_Function_List fn_list = sass_make_function_list(1);
+  // put the only function in this plugin to the list
+  sass_function_set_list_entry(fn_list, 0, fn_foo);
+  // return the list
+  return fn_list;
+}