new cmd line option for specifying the relative path of the dir where Localizable.strings file resides


i have closed the pull request https://github.com/appium/appium/pull/2388 and added the following. The changes are simple & straightforward.

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Some iOS apps do not use single Localizable.strings,but the strings are distributed in many files. 
In this case, one can collate all *.strings files into a single Localizable.strings file, which can be stored somewhere.

This new command line option helps to specify the relative path of the directory for such consolidated Localizable.strings. it defaults to 'en.lproj'. it also accepts ".." to use  the consolidated in the parent dir of .app. 
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