Allow arbirtrary assignment of columns from gene file (--gene_cols)

[pjbriggs]

User requests the ability to assign arbitrary columns from the input gene file to the ID, chromosome, start, end, strand and DE_flag fields.

This could be done via a new --gene_cols option, which would be similar to the existing --peak_cols option for the input peak file (http://rnachipintegrator.readthedocs.io/en/latest/inputs.html#peaks-data-file).