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 {