Make the mock extensible in a similar way as the real redis client

[kamiljano]

As discussed in #157 the redis-mock needs to be extensible via addCommand() and addMockCommand().

The addMockCommand() allows to define new logic for the mock in the following manner:

const mock = require('redis-mock');

mock.addMockCommand({
   'json.set': (client, args, callback) => {
      // your logic here... probably something like: client.set(client, args[0], JSON.stringify(args[1]), callback);
   }
   'json.get': (client, args, callback) => ...
});

None of the mocked functions are added to the mock client straight away though. They get attached to the client only after calling the addCommand() method, which matches the interface of the redis module. So in this case, the addCommand() would have to be called twice:

mock.addCommand('json.set');
mock.addCommand('json.get');

these 2 calls attach the predefined mock definitions into the mock client under 4 name variations:

• mock.JSON_GET
• mock.json_get
• mock['json.get']
• mock['JSON.GET']

Should the addCommand() be called with a parameter that does not have a mock definition, what should be linked to the attached method is a functions throwing an error stating "No mock definition provided for ${commandName}"