Add ability to add multiple sheets to a workbook

[elliotstoner]

There have been a few previous PRs for adding multi-sheet functionality - this is actually built off of davidryansoftware's fork, but with a few tweaks. I'd love feedback and suggestions, and hopefully we can finally get this merged in!

Some key things from feedback previously given:

• File size increases 28% from 6.0KB to 7.7KB for module.js; 5% increase for standalone.js
• Is backwards compatible: allows for either a sheet or sheets property
• sheets is an array (compared to an object in @davidryansoftware's fork)
• Adds a new property sheetname to the Sheet object; using either the sheet or sheets property, you can specific the name you want for each sheet. If a sheetname is not provided, it defaults to Excel's default naming: "Sheet1", "Sheet2", etc.
• The Relationship ID rId was previously hardcoded to rId3 (not sure why it was offset by 2). After some research, I found that Excel defaults to having relationship IDs start at 1, so I reflected that change in this code

Using the new sheets property, the config would look like:

const config = {
    filename: 'Test',
    sheets: [
        {
            sheetname: 'Test Sheet 1',
            data: [
                [{
                    value: 'Sheet 1 Value',
                    type: 'string'
                }, {
                    value: 1234,
                    type: 'number'
                }]
            ]
        },
        {
            data: [
                [{
                    value: 'Sheet 2 Value',
                    type: 'string'
                }, {
                    value: 9876,
                    type: 'number'
                }]
            ]
        }
    ]
};

[ghost]

@elliotstoner Nice Work. I actually agree that sheets should be an array, The code I was working with at the time I created the fork was expecting an object.

[elliotstoner]

Just thought of this - if we like this approach, do we want to have the goal of ultimately getting rid of sheet in favor of sheets in a future major release? If so, do we want to introduce a deprecation warning for sheet now? I would also update the docs to reflect any decisions.

[elliotstoner]

Bump @egeriis. It's been almost 9 months - do we want to move forward with this or are we abandoning it? If the latter then I'm going to branch this off onto my own package

[elliotstoner]

This repo seems unmaintained. I'm going to close this pull request in favor of migrating to xljsx-lite! For anyone looking for this functionality, feel free to check out my repo with these improvements, with more to come! There's two versions: v1.0.2 is the closest to this current repo, while v2.0.0 has some additional improvements (automatic typing, even further reduced bundle size). I look forward to feedback about how we can improve this together!