Let vs const

You are using `let` a lot throughout your project. However in some cases ([here](https://github.com/FACN3/umm-el-fahem/blob/master/src/handler.js#L65) and [here](https://github.com/FACN3/umm-el-fahem/blob/master/src/handler.js#L113) for example) you could just use const. I don't think the variable is reassigned therefore it should be a const.