Enable index name customization

[kentico-ericd]

Motivation

Implements #22

Checklist

• [x] Code follows coding conventions held in this repo
• [x] Automated tests have been added
• [x] Tests are passing
• [x] Docs have been updated (if applicable)
• [x] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

Manually register Algolia indexes using the new approach described here, e.g.

protected override void OnPreInit()
{
    base.OnPreInit();

    var environment = ConfigurationManager.AppSettings["Environment"];
    Service.Use<IAlgoliaIndexRegister>(new DefaultAlgoliaIndexRegister()
        .Add<AlgoliaSiteSearchModel>($"{environment}-{AlgoliaSiteSearchModel.IndexName}")
    );
}

and ensure that the indexing/search functionality is working as expected. Indexes using the RegisterAlgoliaIndex attribute should still work as expected.