Add an ES2018 Target and Lib

[weswigham]

This should include any features we support which are slated for the 2018 spec release (currently stage 4 proposals), or things we think are likely to be in it (ie, promising stage 3 proposals).

[mhegazy]

PRs welcomed if someone wants to get to this before @weswigham does.

[benbraou]

I will provide a pull request for this. First contribution incoming ^^