Allow multiple blocks in directive options

Hi,

I am using Laika to implement a reStructuredText processor with a custom directive that looks like:

    .. newslide::
       :title: The title
       :layout: //the layout is CSS-like
            slide#this p[1] { anchor:(1,2); width:50 }

            slide#this ul[1] li { }

I want to be able to have blank lines inside the `layout` option; this PR removes the `endsOnBlankLine` argument from the `indentedBlock` parser. This makes the behavior closer to the field list parser in `laika.parse.rst.ListParsers.scala`. With this change I get the desired result from parsing the above directive, e.g. the value of the 'layout' option is a String containing both of the indented paragraphs.

All of the tests are passing in Scala 2.10.2 (except for `SectionNumberSpec`, because of a non-existing file that I assume is due to be added to the test resources eventually). The current HEAD doesn't compile on 2.9.3.