add Context generic type

src/tests/00-basic.ts

@@ -1,12 +1,13 @@
 import { Machine, State, On, Do, Goto, Spawn, Unspawn } from '../index';
 
-const beginTimer = ()=>{};
+const beginTimer = (ctx:C, e:E)=>{};
 
 type S = 'green' | 'yellow' | 'red';
 type E = ['entry',null] | ['timer-finished',null];
+type C = null;
 
 const machine = 
-  Machine<S,E>(
+  Machine<S,E,C>(
     State('green',
       On('entry', 
         Do(beginTimer)