v0.8.0

1. change interface Constraint to (name, data, messages, options) => errors, and now it can be applied to both mappings and FormBinder (!!!WARNING: broken change for extension developers)
2. merge/change interfaces PreProcessor/BulkPrePrecessor to (prefix, data, options) => data, and now it can be applied to both FormBinder and mappings (!!!WARNING: broken change for extension developers)
3. rename pipe_: to >-: (!!!WARNING: broken change for normal developers)
4. change pre-processor expandJsonData(sourceKey[, destPrefix]) to expandJsonString([sourceKey][, destPrefix]), and now it can be applied to all mappings and FormBinder (!!!WARNING: broken change for normal developers)
5. add constraint numArrayIndex, used to validate illegal array index, like aaa[t123]
6. enhance constraint required, and now it can be applied to both field mappings and group mappings
7. add built-in field mapping json4s
8. add InputMode to help restrict a Constraint/PreProcessor can accept one input string or multiple input strings or both (for extension developers)