Allow partial tag list to be retrieved

[ArnaudBenassy]

The idea is to avoid full retrieval of tags if not necessary.
In my case, it was necessary to avoid loading of a corrupted Image attached in APIC, that was causing application to crach unexpectedly with certains MP3s.

No BC break, readAllTags continue to work as expected, redirecting to readTags with list from Id3Tags::getId3Tags()

Attached test is running a different Id3TagsReader instance to permit partial load.

[shubhamjain]

Seems ok but why $tagsToRetrieve needs to be an associative array. Doesn't a simple list make more sense? And believe, the two if conditions in the while loop could be optimised and made more cleaner.