Support for Excelx Exporter

[athityakumar]

As mentioned in issue #43, gems like rubyxl / axlsx can be used for exporting to .xlsx format.

[athityakumar]

@zverok - I'll start working on this by tomorrow. It'd be helpful if you could explain what arguments / specific use-case semantics you have in mind. I'm thinking of having the same arguments as the Excel Exporter, but I'm yet to check out on how the xlsx parsing gems have implemented styling.

[athityakumar]

I started implementing this with axlsx, only to realize at the end that axlsx gem hasn't had a new release in the past 2 years. (Check this issue)

Rubyxl is the way to go - I'm re-writing the exporter now.