Connection URL support for Cluster constructor

[onichandame]

Problem

Currently the Cluster constructor cannot take a url string as input. Therefore the user has to manually parse a url string obtained from the runtime environment into a structured argument which the ioredis.Cluster understands.

Desired Behaviour

It would save a lot of repetitive and error-prone work if the library itself takes care of the parsing.

[luin]

The only blocker I can think of for this is to find a URI spec for multi-endpoint.

[onichandame]

@luin I believe a single endpoint url can be used to access a cluster. Multi-endpoint url spec and url parsing for cluster are related but not entirely overlapping.