Add Exists() method

Signed-off-by: Vishal Rana <vr@labstack.com>

context.go

@@ -88,9 +88,12 @@ type (
 		// Set saves data in the context.
 		Set(string, interface{})
 
-		// Del data from the context
+		// Del deletes data from the context.
 		Del(string)
 
+		// Exists checks if that key exists in the context.
+		Exists(string) bool
+
 		// Bind binds the request body into provided type `i`. The default binder
 		// does it based on Content-Type header.
 		Bind(interface{}) error
@@ -292,6 +295,11 @@ func (c *context) Del(key string) {
 	delete(c.store, key)
 }
 
+func (c *context) Exists(key string) bool {
+	_, ok := c.store[key]
+	return ok
+}
+
 func (c *context) Bind(i interface{}) error {
 	return c.echo.binder.Bind(i, c)
 }