Enhancements in XLSX Importer

[athityakumar]

• Parse index (row names)
• Handle merged cells (both order and index)
• Handle formulas in xlsx file

[zverok]

What's the state of it? (It has "ready" label)
What's the state of XLSX exporter? We really need that for nice showcases and stuff.

[athityakumar]

The "ready" label added by Waffle, is when an issue doesn't require further discussions and can be tackled by contributors. All the features in the check-list are yet to be added.

The XLSX Exporter is yet to be worked upon. Opened #63 for tracking that.