fix(orderBy): support sorting by value

Fixes issue #4579, adding support for:

`<p ng-repeat="item in items | orderBy">{{item}}</p>`
`<p ng-repeat="item in items | orderBy:'+'">{{item}}</p>`
`<p ng-repeat="item in items | orderBy:'-'">{{item}}</p>`

It seems unnecessary and redundant in this particular scenario to support the `reverseOrder` parameter and taking `sortPredicate` as an array, so they are ignored (i.e. the following):

`<p ng-repeat="item in items | orderBy:'+':false">{{item}}</p>`
`<p ng-repeat="item in items | orderBy:['+']">{{item}}</p>`