add NSDictionary subtitles support

[alexookah]

I am using this library to get the captions from youtube url while parsing the XML caption youtube provides.
Creating a NSdictionary and using the NSDictionary in the library.

Should i make a pull request for this to be able to use a NSDictionary ?

[mhergon]

Feel free to make PR. I will check and test and evaluate if add to library.

Thanks

[alexookah]

created a pull request. check it out.