feat: preservedSelectors option

[cossssmin]

This PR introduces a new option, preservedSelectors, which is an array of strings representing CSS selectors that should be preserved in the <style> tag when either removeStyleTags or removeinlinedSelectors come into effect.

For example, this:

juice(`
  <style>
    div { color: red; } 
    .preserve-me { background: blue; }
  </style>
  <div class="preserve-me">Test</div>
  `, 
  { 
    removeStyleTags: true, 
    preservedSelectors: ['.preserve-me'] 
  }
)

... would return this HTML:

<style>
  .preserve-me { background: blue; }
</style>

<div class="preserve-me" style="color: red; background: blue">Test</div>