Improve PurgeCSS integration

- Add better Pug support
- Add "modes", with "all" and "conservative" by default
- Allow passing options through to PurgeCSS
- Rename `paths` to `content` to match PurgeCSS

Author: adamwathan

__tests__/fixtures/purge-example.html

@@ -15,7 +15,7 @@
 <div class="min-h-(screen-4) bg-black! font-%#$@ w-(1/2+8)"></div>
 
 <!-- Pug -->
-span.inline-grid.grid-cols-3
+span.inline-grid.grid-cols-3(class="px-1.5")
   .col-span-2
     Hello
   .col-span-1.text-center