Allowing variables (constants) in attributes

[socialblogsite]

This is my first pull request ever. Excuse all my mistakes (you are welcome to point them out though).

Now properties can be expressed as variables, which will use the constants defined in the init.

e.g.
val(--constantname) = _constantname

Example usage:
data-top="margin-top: val(--constantname)px;"

[Prinzhorn]

This needs further discussion. It mixes two completely different concept (I would not reuse the constants option for something which is semantically completely different). See also #69