Add support for extra-chroms (string chromosomes)

[bunop]

Dear @mfranberg ,

I found some time to try to change how your library read chromosomes in order to support string chromosome. I've changed code and tests accordingly and seems to me that I can read my string chromosomes properly. I've also updated the README section, since it seems to me it was a little outdated.

If you could revise this changes and give me a feedback, will be very helpful.

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This PR will close #29