Introduce stricter default limits

[LinusU]

Many limits are currently defaulting to unlimited which isn't that smart at all. I think that we should gather some data on what other libraries are doing and then figure out some good, rather restrictive limits.

I don't see a problem with keeping the low since they are very easy to increase for the user, and since the error message is really clear when a file is to big.

[ryhinchey]

@LinusU I did a little investigation and here's a breakdown of what I found:

Limits
fields: multiparty: 100, node-formidable: 1000
fileSize: multiparty: Infinity, node-formidable: 200mb

https://github.com/node-formidable/node-formidable/blob/master/src/Formidable.js#L14-L22
https://github.com/pillarjs/multiparty/blob/master/index.js#L61-L63

If you're still interested in adding stricter default limits, I'd be happy to make a PR

[LinusU]

Thanks! I wish I'd thought of this issue a bit sooner, since this is a breaking change...

I'll try to add this to #399 asap!

[ryhinchey]

@LinusU I made a PR for the change #848