Add various Notification template generator improvements

- Add ability to set arbitrary value for some placeholders (Fix #133)
- More Unit tests
- Improve doc

src/main/java/io/kamax/mxisd/config/threepid/medium/GenericTemplateConfig.java

@@ -77,6 +77,7 @@ public class GenericTemplateConfig {
     private String invite;
     private Session session = new Session();
     private Map<String, String> generic = new HashMap<>();
+    private Map<String, String> placeholder = new HashMap<>();
 
     public String getInvite() {
         return invite;
@@ -98,4 +99,12 @@ public class GenericTemplateConfig {
         this.generic = generic;
     }
 
+    public Map<String, String> getPlaceholder() {
+        return placeholder;
+    }
+
+    public void setPlaceholder(Map<String, String> placeholder) {
+        this.placeholder = placeholder;
+    }
+
 }