Exclude selectors

Adds a method to exclude certain selectors from being changed by Emogrify.
I use this in an online webpage editor to make sure specific mixed-in UI parts remain untouched because they are styled otherwise.

css
`div { color: red }`

html
`<div><div class="exclude"><div>Test</div></div></div>`

result normally
`<div style="color: red"><div class="exclude" style="color: red"><div style="color: red">Test</div></div></div>`

result after `addExcludeSelector('.exclude')`
`<div style="color: red"><div class="exclude"><div style="color: red">Test</div></div></div>`

result after `addExcludeSelector('.exclude *')`
`<div style="color: red"><div class="exclude" style="color: red"><div>Test</div></div></div>`

result after `addExcludeSelector('.exclude *,.exclude')`
`<div style="color: red"><div class="exclude"><div>Test</div></div></div>`