Implement basic folding for the gherkin editor

This was a bit more cumbersome than I had hoped for, since the
parser/formatter from the gherkin library does not provide the end of
Feature's, Scenario's etc. Hence calculating that using a stack manually.

Other than the ReconcilingStrategy this is largely just boilerplate based
on http://www.eclipse.org/articles/Article-Folding-in-Eclipse-Text-Editors/folding.html

The folding allows to collapse/expand Feature, Scenario, Scenario Outline,
Background and the Examples in Scenario Outlines.

By using the Gherkin Lexer and its listener interface directly one could
possibly support folding multi-line comments, descriptions and multi-line
strings but that may also need more bookkeeping.

fixes #64