Added option to disable removing of optional attribute quotes

I am currently in a situation where I'm minifying html snippets where we use placeholders in attributes which get replaced later on. Because the quotes are optional based on the content of the attribute, which isn't yet known when minifying, the quotes are removed when they shouldn't be. I've added an extra option to disable the removing of quotes completely. 

PS: I just noticed my editor removed some trailing whitespace, which ads some noise to the commit. I hope this isn't a problem.