Re-enable problematic rules

[sindresorhus]

I had to disable (1091fda) some problematic rules in the default config until we manage to fix some issues with them:

custom-error-definition

• custom-error-definition rule throws #150
• False positives in the custom-error-definition rule #90
• checking other problems(fisker)

no-fn-reference-in-iterator

• False-positives in no-fn-reference-in-iterator rule #121
• enable in Rewrite no-fn-reference-in-iterator #666

no-unsafe-regex

• The no-unsafe-regex rule is annoying #153

[fisker]

I'm going to fix no-fn-reference-in-iterator next, when I have time.

[sindresorhus]

Closing in favor of #680.