Add identity and ignore container blocks

[cromo]

As mentioned in the comments of #15, two container blocks could be very handy - especially when paired with the ability to move and fold their contents (#17 and #18).

The identity container would pass through its contents to the code generator unaltered and undecorated. At first glance this wouldn't be very useful, but considering that it can be used to group code and move it around and fold it, could be useful. The idea is similar to a #region in some more traditional languages like C#.

The ignore container would do even less - it would return nothing, effectively commenting out its contents. This is useful for debugging, where code can be temporarily enabled while testing and disabled later without deleting it. It provides a group for such code so that it can be moved or folded as a unit.

Together, switching back and forth between an identity and an ignore container gives a compile-time enable switch for a block of code (or even a single token!), which could be quite useful.