Make browsers list configurable

[cr101]

Autoprefixer's browsers option is currently hardcoded. This PR adds the browserslist key in package.json which allows developers to set different browsers for both production and development.
Autoprefixer which relies on Browserslist will find its config automatically.
This PR fixes #2248 and #892

[gaearon]

Does this let me specify browserslist in package.json of unejected apps? We should support that (with our hidden default if it’s unspecified).

[cr101]

@gaearon

Does this let me specify browserslist in package.json of unejected apps?

Yes. Running create-react-app my-app will add the default browserslist in package.json as below:

[gaearon]

Do you think it would make sense to use a single list by default, and document splitting dev/prod in user guide? Not sure if it would be confusing to most people or not.

I also think we should still provide a default for people who don't have a browserslist in their package.json. Since they probably will forget to add it upgrading. We should either force it to be there, or provide a good default.

[cr101]

@gaearon I agree that if people forget to add browserslist to their package.json when they upgrade then we should force it to be there. If you agree with the approach then I will do the user documentation next.

Do you think it would make sense to use a single list by default, and document splitting dev/prod in user guide? Not sure if it would be confusing to most people or not.

A number of people have asked to split dev/prod

[gaearon]

I'd like to split these changes if you don't mind.

Let's start with using default values when browserslist is not specified. We can force it to be specified in a later release, but I'd rather release this as a non-breaking change for now.

I would also like to support both single array and an object with development and production configurations.

[gaearon]

I'd like to not introduce warnings for this.
It will be a straightforward step in migration guide, but for now I don't want people to get annoyed.

[cr101]

Sorry, I accidentally messed up committing my changes while merging changes from upstream.

[gaearon]

Hmm. I wonder if we should count this as a breaking change or not. I guess not.

[Timer]

IIRC, browserlist searches all the way up the folder tree. This might cause confusing situations if users have dangling package.jsons (something we've seen before) or malformed ones (it complains loudly when malformed).
Can we limit its search scope?

[cr101]

@Timer Any idea how to limit the browserlist search scope?

[viankakrisna]

I think this can be moved to a separate module to avoid DRY, maybe config/getSupportedBrowsers?

[Timer]

@cr101 https://github.com/ai/browserslist/blob/master/index.js#L361 looks like it can't be configured; open an issue and PR maybe?

[cr101]

@Timer browserslist just does a JSON.parse of package.json to find the list of browsers section.

[Timer]

What needs done here?
Have we made that pull request to limit the scope searching?

[cr101]

Have we made that pull request to limit the scope searching?

I don't believe so. I'm now on Windows which makes it tricky to test the changes.
@Timer I'd appreciate your help with limiting the search scope.

[gaearon]

Really sorry about this—we were very busy with React 16 rollout and by the time we got around to reviewing #3644 was more fresh so we took it instead. Thanks for prototyping this early on!