* Chore: add a fuzzer to detect bugs in core rules

This commit adds a fuzzer to detect bugs in core rules. The fuzzer can detect two types of problems: crashes (where a rule throws an error given a certain syntax) and autofix errors (where an autofix results in a syntax error).

The fuzzer works by running eslint on randomly-generated code with a random config. The code is generated with [eslump](https://github.com/lydell/eslump), and the config is generated with the existing autoconfig logic.

The fuzzer can be run with `npm run fuzz`. Eventually, I think we should add the fuzzer to the normal `npm test` build.

* Pin eslump version

* Update linter logic with new APIs from ESLint 4

* Upgrade eslump to 1.6.0