Add --silent option for a quieter UI

This addresses #2423.  Passing `--silent` to any command will set the UI will only write errors. This allows XML output from `ember test` to be captured in a file without any additional text (printed ember version, pleasent progress, etc.) 

To achieve this, I introduced the concept of a write level / log level to the UI object. `UI.write` and `UI.writeLine` now accept a second argument that is the write level. If the write level set for the UI is higher the write level passed to `UI.write`, nothing will be printed. By default, the write level for a `UI` object and for `UI.write` and `UI.writeLine` is `'INFO'`. The write levels, in order, are: `'DEBUG'`, `'INFO'`, `'WARNING'`, `'ERROR'`. 

This is my first time working in node / on ember-cli—if the implementation is off let me know how I can change it. I would be happy to add tests if all looks well.

good call, could never spell that right anyway