Implement template option

[ProchaLu]

Closes #30

Description

This PR enhances the library by adding a template option, improving its usability and testing capabilities. By adding this feature, developers can now easily include custom templates, such as data-test-id attributes, for better testing purposes.

• Code without the use of the template option:

<NextTopLoader />

• Code with the use of the template option:

<NextTopLoader 
  template='<div class="bar" data-test-id="loading" role="bar"><div class="peg"></div></div>
  <div class="spinner" role="spinner"><div class="spinner-icon"></div></div>' />