Whitespace collapse needs a "conservative" (HTML strict) option

[stevenvachon]

The current implementation has forced me to add pseudo-elements containing whitespace in situations like this:

<h3>Title</h3>
<ul>
  <li>Item</li>
</ul>

...minifying to:

<h3>Title</h3><ul><li>Item</li></ul>

...which, when I have:

h3, ul, li {
  display: inline;
}

h3::after {
  content: ':';
}

...renders as:

Title:Item

...instead of:

Title: Item

...which is how spec-compliant HTML collapses whitespace. Obviously, my fix is:

h3::after {
  content: ': ';
}

...but this isn't noticed in dev because of #78 and had to be corrected after a deploy.

Regardless, this plugin should optionally use normalize-html-whitespace.