Ability to add multiple sheets

[IHIutch]

Essentially this does the same as the previous plugin version, but now you can add the spreadsheets as an array of objects to load the data from each spreadsheet.

Another thing to note: I changed the name of sheetId to spreadsheetId so it was closer to the google specs and since sheetId is already a thing (which maybe could be iterated through as well).

I can also update the readme.

[cselex]

Please merge this feature

[IHIutch]

I updated this pull request to align with some of the more recent updates. I also added some examples I thought would be helpful in the readme as well as correcting a minor inaccuracy.

This PR does a few things, first and foremost it adds the ability to add multiple spreadsheets and/or sheets at the same time. Secondly, it forces users to choose a "Collection Name" which I think makes adding this data a little more intentional. Finally, it adds an "ID" key to so that the data doesn't need to rely on GraphQL to generate an ID. This new ID parallels the rows of a user's spreadsheet which can be leveraged for sorting and organization (as well as comparing data visually).

Personally, I'd remove the "Using Templates" section of the ReadMe, I think the Gridsome docs provide an excellent example of how to use Templates, and I think that would require less maintenance from you.

I think being able to add multiple spreadsheets/sheets really makes this plugin powerful and I'd love to see this merged.

Happy to get any feedback and update as necessary!

[dmitrybubyakin]

@IHIutch Thank you!

[IHIutch]

Bump