Fixed #79: Curly quotes present in code listings under XeTeX engine

* Used non-curly quotes in listings (`U+0022`) with XeTeX engine instead of `U+201D`.
* Updated example Markdown and example PDF to emphasise smart-vs-dumb quotes.
* Added comment that a fix for this workaround is already available but not released.

eisvogel.tex

@@ -562,6 +562,25 @@ $if(mainfont)$
 $else$
 \usepackage[default]{sourcesanspro}
 \usepackage{sourcecodepro}
+
+% XeLaTeX specific adjustments for straight quotes: https://tex.stackexchange.com/a/354887
+% This issue is already fixed (see https://github.com/silkeh/latex-sourcecodepro/pull/5) but the 
+% fix is still unreleased.
+% TODO: Remove this workaround when the new version of sourcecodepro is reelased on CTAN.
+\ifxetex
+\makeatletter
+\defaultfontfeatures[\ttfamily]
+  { Numbers   = \sourcecodepro@figurestyle,
+    Scale     = \SourceCodePro@scale,
+    Extension = .otf }
+\setmonofont
+  [ UprightFont    = *-\sourcecodepro@regstyle,
+    ItalicFont     = *-\sourcecodepro@regstyle It,
+    BoldFont       = *-\sourcecodepro@boldstyle,
+    BoldItalicFont = *-\sourcecodepro@boldstyle It ]
+  {SourceCodePro}
+\makeatother
+\fi
 $endif$
 
 %

examples/listings/listings.md

@@ -10,8 +10,8 @@ keywords: [Markdown, Example]
 
 ## Aesculeae domus vincemur et Veneris adsuetus lapsum
 
-Lorem markdownum Letoia, et alios: figurae flectentem annis aliquid Peneosque ab
-esse, obstat gravitate.
+Lorem markdownum Letoia, et alios -- "figurae flectentem annis aliquid Peneosque" ab
+esse, 'obstat' gravitate.
 
 ```java
 public class Example implements LoremIpsum {