Mutations & More parameters to listeners

Hi,

I worked on a little [PR](https://github.com/developit/unistore/pull/161) adding mutations

The implementation is small and optional to usage, however enabling mutations is radical in a sense that they disable the standard system. (setState works as intended tough, there are plenty more detail in the PR comment)

I'm not sure this should be merged tough, I just wanted to share the code, even if there are missing tests I wondered what feedback it could get

Thanks a lot for allowing all this fun :)