document new approach
rework erb preprocessing logic to correctly handle statements

the new approach is tokenize on any erb statement/expression, and then:
1. whenever we find a t block expr, go till we find the end
2. if we find another t block expr before the end, goto step 1
3. capture any inline expressions along the way
4. if we find *any* other statement or block expr, abort, since it's a no-
   go

still need to actually implement step 3
erb expression -> placeholder inference