Offset option for excelx #each_row

[ignat-z]

I have not found any reference about such ability but can't imagine how are you living without it. So, please, correct me in case if I re-invited wheel.
It can be useful for files with known structure or files with insignificant information on first rows

Thanks

[coveralls]

Coverage increased (+0.01%) to 93.64% when pulling 8bb5517 on ignat-zakrevsky:offset-feature-for-excelx into 38d708d on roo-rb:master.