Add support for customizing the button label in ShowMoreLoaderResult

[juliomoreira]

Former issue: #129

Describe the solution you'd like
We would like to have the ability to dynamically set the ShowMoreLoaderResult button label possibly giving the user feedback of how many items are showing and how many items are left.

Describe alternatives you've considered
Right now the only option is to set the label in translation files but it doesn't seems to support dynamic content.

Additional context

[denisebiscaro]

vtex-apps/search-result#222