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'd prefer to keep it simple and assume we're not going to add a lot of options. So doing `var force = opts.force !== false` to default an option is preferred over exposing a function to create a config object.