Modify add_xcode_base_configuration to respect existing base
configurations:
 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.
[Changelog] Add link to issue
[XCConfigIntegrator] Update descriptions to be more verbose

Thanks @alloy!
Merge pull request #2581 from CocoaPods/use_base_configuration

Modify add_xcode_base_configuration to respect existing base configurations