Merge default options with config files if property not specified

[justcfx2u]

At present, most mosca config file options do not specify a logger object.
That makes them break when, for example, trying to use verbose output mode.

Example mosca.conf:

module.exports = {
  port: 1883,
  backend: {
    type: "redis"
  }
};

Then run:

mosca -c ./mosca.conf -v

Will dump an error like:

$ mosca -c ./mosca.conf -v

/usr/lib/node_modules/mosca/lib/cli.js:96
      opts.logger.level = 30;
                        ^
TypeError: Cannot set property 'level' of undefined
    at /usr/lib/node_modules/mosca/lib/cli.js:96:25
    at cli (/usr/lib/node_modules/mosca/lib/cli.js:232:12)
    at Object.<anonymous> (/usr/lib/node_modules/mosca/bin/mosca:3:22)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:901:3

This patch makes sure all "default" options are set to default values (merging default options with those specified in a config file) instead of outright replacing them, allowing configuration files to override only what is specified, while having a reasonable guarantee that the rest of the code which may depend on a default value can continue to do so. It also has the side-effect of making the current examples work better.

[mcollina]

Thanks! Could you please add a simple unit test in 'test/cli_spec.js' to test this behaviour? So we won't break it in the future.

[justcfx2u]

Done. Even though the test case is rather specific it manages to illustrate that the above patch is doing something desirable.

[mcollina]

That's perfect! Thanks a million.

[mcollina]

Released as 0.12.1.