Support ES5 and ES6/ES2015 projects

[nunofgs]

This repository is only useful for ES6 projects since there are multiple rules that enforce ES6-only features.

I'd like to start a discussion regarding solving this issue for users who are still maintaining ES5 projects.

The way I see it, we have a few options:

1. We can keep it as is and update the README.md to make it clear we do not support ES5 projects.
2. We can split this repository into two:
   • jscs-config-seegno-es5 would contain the base rules.
   • jscs-config-seegno-es2015 would contain the ES2015-specific rules and would extend the es5 repo.
3. We can update the README.md to instruct users on how to disable all ES6 rules if needed.

WDYT?

[ruimarinho]

I think we won't be working on ES5 projects anymore, so we should make an effort to upgrade them instead. WDYT?

[fixe]

+1 for ditching ES5.