remove 'jspm_packages' by default

[la-yumba]

since it's smart enough to avoid linting node_modules and bower_components, let's also remove jspm_packages

[feross]

We don't currently ignore bower_components. This is a slippery slope. I don't want standard to be responsible for ignoring every new alternative package manager's folder name. You can ignore folders manually by following these instructions in the readme.

[chenxsan]

@la-yumba If you are using any editors like VS Code with standard extension, you can ignore those jspm_packages globally by configuring settings.json instead of configuring every package.json of your projects.