Allow client to enqueue a task with unique option

Changes:

- Added Unique option for clients
- Require go v.13 or above (to use new errors wrapping functions)
- Fixed adding queue key to all-queues set (asynq:queues) when scheduling.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `Unique` option was added to allow client to enqueue a task only if it's unique within a certain time period.
+
 ## [0.6.2] - 2020-03-15
 
 ### Added