Function getFirst

Current:

const author = importer.getAllItems<Person>(config.owner)[0];

After:

const author = importer.getFirst<Person>(config.owner);

Todo:

• Add getFirst function into IImporter interface (not in IImporterLegacy)
• Implement getFist in a Importer class (not in ImporterLegacy)
• Update sample in Readme (use getFist)

The worksheets received from the Excel file do not keep the order.

...
{:ok, lists} = Xler.worksheets(file_path)
Logger.debug("lists: #{inspect(lists, pretty: true)}")
...

And run it a few times, result:

13:37:49.942 [debug] lists: ["Sheet2", "Sheet1", "Sheet3"]
13:37:49.942 [debug] lists: ["Sheet3", "Sheet2", "Sheet1"]

The order of the worksheets can be impor