[Mapcss] Little syntax easement

[Richard Fairhurst]

Hi all,

I'd like to suggest a little 'easement' to make MapCSS syntax less 
verbose in one particular case.

Right now, if you want to use a tag value in a declaration, you need to 
fire up the eval parser:
	way[highway] { width: eval("tag('lanes')"); }

It'd be more pleasant, and more efficient, to allow this too:
	way[highway] { width: tag('lanes'); }

and it's quite CSS-like, because you can of course use a similar syntax 
in Real CSS:
	way[highway] { color: rgb(0,0,0); }

cheers
Richard