feat: support optionally passing kong.Context to Validate()

Fixes #340

README.md

@@ -663,8 +663,7 @@ func main() {
 ## Validation
 
 Kong does validation on the structure of a command-line, but also supports
-extensible validation. Any node in the tree may implement the following
-interface:
+extensible validation. Any node in the tree may implement either of the following interfaces:
 
 ```go
 type Validatable interface {
@@ -672,6 +671,12 @@ type Validatable interface {
  }
 ```
 
+```go
+type Validatable interface {
+    Validate(kctx *kong.Context) error
+ }
+```
+
 If one of these nodes is in the active command-line it will be called during
 normal validation.