Add await as future reserved keyword

[danez]

await is (similar to enum which is already part of esutils) a future reserved keyword in ES6
http://www.ecma-international.org/ecma-262/6.0/#sec-future-reserved-words

This came up in babel/babel#4198 and babel is using esutils to get the reserved keywords.

[danez]

Friendly push :)
@michaelficarra

[jquense]

friendly ping again :) would love to see this merged

[michaelficarra]

await is only treated as a FutureReservedWord when Module is the goal symbol of the syntactic grammar.

var await is a valid Script. Having this function say that await is unconditionally a FutureReservedWord would be wrong. We could possibly add another parameter, moduleIsTheGoalSymbol/isModule.