Add options, fix a bug and add some outputs

1. Adding two options (README.md is updated) : 

- cache (use cache or not)
- force (force resinstall or not)

2. When using `napa username/repo`, the repo will be ignored if a `package.json` exists in cwd. I fixed that.

3. I added some outputs to tell if repo is ignored or reinstalled. Track log.info() statements to see what I have add.

Hope you'll like it.

I did not add test statements, because I only know jasmine by now and I did not recognize the langage you used. Tell me and I will try to write some.
I understand your point of view. 

I agree about `method` property which is only needed for test relevance. I just wanted to do it as nice as I could. 

But adding options to a module always brings API modifications. You need to add a parser with its own API. You also need to define the default values, and the way to forward them from the client (`nada`) to the real engine (`Pkg`). 
You can do the job with monkey patches, but it is pretty uggly and less maintanable...

Except the `nada.args` I refactored to fix the issue https://github.com/shama/napa/issues/12, I did not think I wrote to much code to add an "options" functionnality.