Add support for extra-chroms (string chromosomes)

[bunop]

Dear all, I don't know if it possible or easy to implement at the current state of art of this library: will be possible to add support for the plink --allow-extra-chr option (more precisely deal with string chromosome like X,Y,Contig123456)? I saw in src/bim_parse.c that the chromosome field is converted in a integer value: will be sufficent to change this line and all the other chromosome declaration in code? Many thanks

[mfranberg]

That makes a lot of sense I think. Not sure why I have them as int in the first place knowing very well about X. Lets change it and add some notes for next release saying it is backward incompatible.