Adds a spreadsheet/XLSX/XLS parser and writer. Closes #136

Includes unit tests:

1. Parses the example XLSX file from here: https://github.com/spdx/spdx-spec/tree/development/v2.2.2/examples and then checks the result against a handwritten Document struct
2. Writes out the handwritten struct to a XLSX file, and then parses the written XLSX file to verify that no data was lost and all the values match what was expected.

Depends on and contains commits from #133, #134, and #137

Also bumps the Github Actions workflow to use Go 1.15, because that is the minimum version supported for the Excelize module used for interacting with XLSX files.