added options to configure cache

docs/config.md

@@ -45,4 +45,16 @@ database:
   Path: "./vikunja.db"
   # Whether to show mysql queries or not. Useful for debugging.
   showqueries: "false"
+
+cache:
+  # If cache is enabled or not
+  enabled: false
+  # Cache type. Possible values are memory or redis
+  type: memory
+  # When using memory this defines the maximum size an element can take
+  maxelementsize: 1000
+  # When using redis, this is the host of the redis server including its port.
+  redishost: 'localhost:6379'
+  # When using redis, this is the password used to authenicate against the redis server
+  redispassword: ''
 ```