Support attribute extension

[mattesmohr]

This pull request addresses #182 and adds support for conditional attribute extension, by giving the user the choice of a .replacing or .combining operation, while the modifier defaults to .replacing.

Division {
}
.class("lorem")
.modify(true, use: .combining) {
   $0.class("ipsum")
}

// Renders <div class="lorem ipsum">...

Data types, which cannot meaningfully combined like Bool will automatically replaced. Open for debate, whether a String should count as a combinable type. For now, it is more about list attributes such as class, headers... I would say.

This pull request acknowledges that the value of the attributes like class, headers are actually a list, defined by its individual values plus a separator. With that in place, all information needed for a predictable combination is already available.