Add `trailing commas in func exprs` as a stage 1 current proposal

Per the meeting notes from the Sep'14 meeting, this proposal is
currently at Stage 1.

https://github.com/tc39/tc39-notes/blob/master/es6/2014-09/sept-24.md#51-trailing-commas-in-function-call-expressions-and-declarations
Merge pull request #15 from jeffmo/master

Add `trailing commas in func exprs` as a stage 1 current proposal