spacebars style expression for jade

rel to #81 #80 
rules: you can use a helper inside jade with brackets, you can pass any arguments (i.e. another helpers) inside brackets
```jade
 	+if anyHelper(arg1, arg2, arg3, ...)
 	div(class=anyHelper(arg1, arg2, arg3, ...))
 	h1= anyHelper(arg1, arg2, arg3, ...)
 	h1 #{anyHelper(arg1, arg2, arg3, ...)}
```