Why is eslint-plugin-import configured to ignore node_modules?

In [eslint-config-airbnb-base/rules/imports.js](https://github.com/airbnb/javascript/blob/master/packages/eslint-config-airbnb-base/rules/imports.js#L26), `import/ignore` is set to ignore `node_modules` and a number of non-js files (coffee, css, etc.)

Ignoring `node_modules` means that none of the lint analysis can happen against external libraries. In particular, I care about `import/named` flagging things like

```
import { badBadBad } from 'redux';
```

What's the rationale for this change? We can already use eslint's native ignore functionality to avoid linting all of node_modules, right?

Start with packages/eslint-config-airbnb-base/rules/imports.js at the import/ignore configuration, then review how eslint-plugin-import handles node_modules and non-JavaScript files. Reproduce the reported import/named example with redux and determine whether the existing ignore rules have a documented rationale; done means the rationale or resulting configuration decision is recorded.

Written by the indexing model from the issue text.