Ability to merge ExportOptions.plist from App_Resources and from plugins

It’ll be great to support merging of ExportOptions.plist file from App_Resources and from plugins. This way every user will be able to add custom options to this file and NativeScript CLI will merge them with the default options. Also, every plugin author will be able to add such custom options in platforms/ios folder of the plugin in case when it is required.

Currently NativeScript CLI supports merging of Info.plist files from plugins and from App_Resources. As the format of ExportOptions.plist and Info.plist is the same (the both files are .plist format), it seems the merging of ExportOptions.plist should be very similar to the merging of Info.plist files. Here is the code that merges Info.plist files. The mergeInfoPlists function has hardcoded path to the Info.plist file. It seems the common logic of this function can be extracted to a separate function, for example mergePlistFile. After that we can call this function for Info.plist and ExportOptions.plist:

mergePlistFile(<path to Info.plist>);
mergePlistFile(<path to ExportOptions.plist>);

从 lib/services/ios-project-service.ts 中的 mergeInfoPlists 函数和硬编码的 Info.plist 路径开始。跟踪 plugin 和 App_Resources plist 文件的定位与合并方式，然后确定同一个入口点应如何涵盖 ExportOptions.plist。完成的标准是两个 plist 源都可以提供自定义的 ExportOptions.plist 选项，同时现有的 Info.plist 合并仍能正常工作。