ERB like trim mode, another approach

I've added separate regular expressions for capturing `-%}` and `-}}` followed by an optional newline character. 
The markup capture in `block.rb` needed to be changed from greedy `(.*)?` to lazy `(.*?)?`.

Furthermore I've changed the `push` method to push the context before raising `StackLevelError` so it can be popped later. `IncludeTagTest` fails otherwise, at least on my system.