Allow conditionnal blocks within template

[ccocchi]

To replace if and unless within template we introduce the condition keyword. It takes a proc (that take the current object rendered) as argument and a block. The block is parsed at compile time.

The source compiled from the block will be called if the proc is evaluated as true at rendering time. Here's a use example

object :@user

attributes :name
condition(->(u) { u == current_user }) do
 attributes :secret
end