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.
Right, that test is why I don't want it. Each API exposed is another facet I'll have to maintain and I don't want to maintain a config parsing method in this library. This PR now adds `config()`, `method`, `knownOpts`, `shortOpts` and `defaultOpts` to the API. As well as `execOpts` and `method` to `pkg.js`. I don't think those extra APIs are necessary to implement `--force` and `--cache` options.

We can assume that `nopt` has tests and will process cli options accordingly as well.