Rewrite parse_block_elements() to remove goto

The `goto` clause is the only thing which prevents this class to work on PHP 5.2.

Also, using goto is not considered a good coding practice and should be avoided since there are other control flow constructs which can replace the need for `goto`.

In the case of the `Paragraph` class, I just put the code which the `goto` skipped before in an `if` and inverted the condition.

All tests are passing.