Adds count formatFn

[manconeg]

This change allows a developer to provide a function which formats the count text as required by their needs. For instance:

<Counter start="1" formatFn={(count) => `${count}%`} />

will display the count like 1%.

Let me know if you have feedback or would prefer a different implementation to achieve this.

[yasaricli]

This will be available in version 1.3.1. You can update and use it once published.

Thanks again for the contribution!