Add support for ignoring certain paths

[cyberixae]

I typically need this kind of tool for linting json configurations of my project. The problem is that it also lints all json files within node_modules.

[educastellano]

That sounds it would be a useful feature, how about:

jsonlint-tree app --ignore node_modules

The recursive-readdir module accepts a second parameter to ignore files, we could pass the cli argument directly here.

If you make a PR I'll be happy to merge 🙂