Add ability to pull worksheet names from an Excel workbook

[FraserGreenroyd]

Issues addressed by this PR

Fixes #27

Test files

Available here - a GH file and an Excel file linked to test with.

[FraserGreenroyd]

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[desaiwangBH]

Great! Tested with my own file and I like the search function, especially as it is case insensitive :)

[FraserGreenroyd]

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