Support checking against user-defined ES versions

[obahareth]

Instead of just checking for ES5 specifically, we should allow users to specify an ES version and default to 5. Although if we start to do that this repository will probably need a rename.

• Come up with a new name.
• Support user-defined ES versions while defaulting to ES5.
• Publish an are-you-es5 release to NPM that displays a notice on an npm install on the rename with the new package name added, but keep existing package and all existing versions to avoid breaking anything.

[jroru]

Related issue: #64

Would be great to see this tool work with browserslist