Allow configuration of default JS file types

Per https://github.com/eslint/eslint/issues/2388#issuecomment-98183996:

Let’s allow configuration of the file suffixes, maybe in the .eslintrc file.

The problem was that I was not including the jsx files at the command line. Almost all my JS files have suffix JSX.

So this works:

#!/bin/sh
eslint . --ext .jsx,.js
jscs .

On a separate note, if team members don’t want to install eslint globally, any disadvantage to doing this:

npm run eslint . -- --ext .jsx,.js

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[nzakas]

We don’t plan to address this until we look at redoing how configuration is done, so no immediate changes are expected here.

[nzakas]

We will look into this after 2.0.0, unless someone wants to investigate in the meantime.