Allow to instantiate a client instance

[daffl]

This allows to instantiate a standalone client service like:

const rest = require('feathers-rest/client');

const todoService = rest(url).fetch(fetch).service('todos');

Also throws an error when configured twice on the client.

[ekryski]

Do you want to add a test for the error being thrown? In the case where you initialize two or more providers?

[daffl]

Should be right in https://github.com/feathersjs/feathers-rest/pull/16/files#diff-e50c7552d4d65d031f80b6c5e56591deR36

[ekryski]

My bad.