Add optional name to replace OTHER_LDFLAGS in xcconfig hash

I've been working on fully integrating cocoapods into my project set up. Since I also use xcconfig files to manage my build settings, I'm wanting to get to the point where all variables in the **Pods.xcconfig** are namespaced.

In working towards a solution (https://github.com/CocoaPods/CocoaPods/pull/1016), it seems easier to provide this flexibility inside **xcodeproj** here, and then use that new optional functionality in cocoapods.

Here, I've added the ability to specify what you want the output linker flags to be instead of `OTHER_LDFLAGS`. It defaults to the same value as before, so all existing behaviour should be unchanged.

On the input side (from a hash or xcconfig file), it still expects `OTHER_LDFLAGS`. Only on the output `to_hash` does it use the optional name.