Allow binding to via the kwarg in tokens

[bradgessler]

This commit provides a shortcut for binding to a class keyword argument and passing it into tokens, like this:

def view_template
  red(class: "buzz")
end

def red(class:)
  div(class: tokens("fizz", class:))
end

Prior to this commit, I'd have to manually bind to the class keyword argument passed into tokens like this:

def red(class:)
  div(class: tokens("fizz", binding.local_variable_get(:class)))
end

[joeldrapper]

This is an awesome idea. I want to do something with it, but I don’t think tokens is the right place for this as it is currently implemented. Keyword arguments on the tokens method are for conditional tokens, where the key is the condition and the value is the token set.