implementation of https://github.com/samtools/hts-specs/issues/100

[lindenb]

Hi , this is a pull request to suggest a new Tag AN for @seq in the sequence dictionary.

Reference sequence Aternative Names. A semicolon separated list of alternative names for this sequence e.g:
1;01;CM000663;NC 000001.10. Tools are free to use this information to decode the users’ posi-
tions.

see also issue id 100: #100

[jmarshall]

Thanks. Closing so we keep discussion in one place: issue #100.