Allow context value to be set after parse

This change allows a context value to be set after parsing. The use case is updating default settings in a Before func.

An example usage:

```
		f, err := os.Open(configPath)
		if err == nil {
			config, err := docli.NewConfig(f)
			if err != nil {
                       panic(err)
			}

			c.Set("token", config.APIKey)
		}
```

context.go

@@ -132,6 +132,11 @@ func (c *Context) NumFlags() int {
 	return c.flagSet.NFlag()
 }
 
+// Set sets a context flag to a value.
+func (c *Context) Set(name, value string) error {
+	return c.flagSet.Set(name, value)
+}
+
 // Determines if the flag was actually set
 func (c *Context) IsSet(name string) bool {
 	if c.setFlags == nil {

context_test.go

@@ -113,3 +113,12 @@ func TestContext_NumFlags(t *testing.T) {
 	globalSet.Parse([]string{"--myflagGlobal"})
 	expect(t, c.NumFlags(), 2)
 }
+
+func TestContext_Set(t *testing.T) {
+	set := flag.NewFlagSet("test", 0)
+	set.Int("int", 5, "an int")
+	c := cli.NewContext(nil, set, nil)
+
+	c.Set("int", "1")
+	expect(t, c.Int("int"), 1)
+}