* Improving variable expansion runtime
The current implementation of variable expansion is O(n x m) with n being the
number of strings in the project spec and m being the number of project variables.
This implementation is now O(n).
Also, this effectively deprecates the support for $legacy_variables in favor of the
${new_variables} making this whole patch possible.
* Adding option to disable variable expansion
* Adding performance test for spec loading
* Updating changelog
This allows parameterizing templates. Also change
placeholder syntax to `${placeholderName}` also for
existing placeholders `$target_name`and `$platform`
and generate warnings when using the old placeholder
syntax.
This allows users to set the defaultConfigurationName project wide. This
setting corresponds to the drop down in the project settings that says
"Use CONFIG for command line builds". This affects which configuration
Xcode looks in for some settings, even if you pass `-configuration FOO`.