Support optional argument types

[jgiles]

Example:

/** @param {ParamType=} opt_param */
var fnOptParam = function(opt_param) {
  opt_param; //: ParamType=
};
fnOptParam; //: fn(opt_param: ParamType=)

Note: This is separate from nullability information.
The Tern doc_comment plugin modifies the displayed parameter names in order to surface this information. Ideally, it would be part of the type information attached to the parameter.