Add custom eslint formatter

[sidoshi]

Replaces hacky code to format eslint errors by using custom loader.

I built this formatter on top of the default stylish formatter.
It omits warnings if there are any eslint errors as required in #2120 and errors and warnings looks the same as earlier.

Closes #2121

[gaearon]

Any reason you closed?

[sidoshi]

While reformatting my code after testing I used absolute require in require ('formatter') instead of relative, like require('./formatter'). Trying to fix it without opening another pull request.

[gaearon]

We probably don't want this formatter to live in react-scripts because we don't want to keep it in user code after ejecting. Let's move it to react-dev-utils?

[sidoshi]

Sure.

[sidoshi]

I have pushed to the branch I have sent pull request from. But the changes doesn't seem to be updated. Is it because I have closed the PR?
Sorry for the inconvenience, I am learning how this things work.

[gaearon]

Yea, could you push to a new branch and open another PR please?

[sidoshi]

Okay doing it. Thanks.

[sidoshi]

See #2138