+ Recognize a double hyphen as an Em-dash, even when it occurs next
to punctuation (e.g. a quotation mark).
+ Collapse space around Em-dashes.
+ Process quotes before dashes. This way (foo -- 'bar') will turn into
(foo---`bar') instead of (foo---'bar').
git-svn-id: https://pandoc.googlecode.com/svn/trunk@49 788f1e2b-df1e-0410-8736-df70ead52e1b