Fix CSS purging

[aklajnert]

This is an attempt to resolve #19. It basically reverts the purging behavior to the one in 0.2.0, which seems reasonable to me.
I can't find any more context around 0.2.1, but classes like .md, .-mx-6 seem to be not used, so it makes sense to remove them.

@fattenap - maybe you remember more about why the behavior was changed in this way in 0.2.1,

Edit: This works slightly better than 0.2.0 in my case. In 0.2.0 a class .py-0\.5 was purged despite being used and now it is correctly retained. However, I doubt that it's because of this change, probably something else between 0.2.1 and 0.2.3,

[MartinKavik]

Let's wait a bit for @fattenap's reply. I don't see any problems on the first glance and if it works with your projects, then it's improved even if there are some hidden edge-cases.

[MartinKavik]

classes like .md, .-mx-6 seem to be not used, so it makes sense to remove them

According to the test and input files index.hbs and input.rs it should be true (result).

@aklajnert Please update CHANGELOG and I'll merge it.

[aklajnert]

@aklajnert Please update CHANGELOG and I'll merge it.

Should I add an entry in the changelog under UNRELEASED, or call it 0.2.4 already?

Edit: Updated as UNRELEASED for now.