fix: web-ext lint: ignore node_modules and other artifacts

[kumar303]

Fixes #397

Tasks:

• merge the linter patch to get a new callback: Add callback to control which files can be scanned addons-linter#862
• hook up the callback
• make sure the babel-polyfill patch (Using linter as a lib: Error: only one instance of babel-polyfill is allowed addons-linter#864) is in the linter
• bump package.json after linter release (feat: Deprecated Node 0.12 for addons-linter support #414)

[coveralls]

Coverage remained the same at 100.0% when pulling 3aeda8a on kumar303:lint-filter into 0c54b95 on mozilla:master.

[rpl]

👍 lgtm, once the updated addons-linter has been released (and the its version in our package.json updated accordingly), I think that this is ready to go.

r+

[pdehaan]

Any chance we're deploying a 1.4.0 release to npm with this fix in?

[kumar303]

@pdehaan sure, I can cut a new release right now

[kumar303]

when this is finished building the release should be on npm: https://travis-ci.org/mozilla/web-ext/builds/149854164