conditions: add interface for adding condition factories (#33)

Signed-off-by: Mikhail Pronyakin <pronyakin@onsec.ru>

README.md

@@ -338,6 +338,22 @@ There are a couple of conditions available:
 * [String Equal Condition](condition_string_equal.go): Matches two strings.
 * [Subject Condition](condition_subject_equal.go): Matches when the condition field is equal to the subject field.
 
+You can add custom conditions by using `ladon.ConditionFactories` (for more information see condition.go):
+
+```go
+import "github.com/ory-am/ladon"
+
+func main() {
+    // ...
+
+    ladon.ConditionFactories[new(CustomCondition).GetName()] = func() Condition {
+        return new(CustomCondition)
+    }
+
+    // ...
+}
+```
+
 ### Examples
 
 Let's assume that we are using the policy from above for the following requests.

condition.go

@@ -55,7 +55,7 @@ func (cs Conditions) UnmarshalJSON(data []byte) error {
 	}
 
 	for k, jc := range jcs {
-		for name, c := range conditionFactories {
+		for name, c := range ConditionFactories {
 			if name == jc.Type {
 				dc = c()
 
@@ -82,7 +82,8 @@ type jsonCondition struct {
 	Options json.RawMessage `json:"options"`
 }
 
-var conditionFactories = map[string]func() Condition{
+// You can add custom conditions to ConditionFactories
+var ConditionFactories = map[string]func() Condition{
 	new(StringEqualCondition).GetName(): func() Condition {
 		return new(StringEqualCondition)
 	},