LaTeX writer: Make Horizontal Rules more flexible

Currently, pandoc has hard-coded the following in order to make horizontal
rules in LaTeX:

```hs
"\\begin{center}\\rule{3in}{0.4pt}\\end{center}"
```

Which is fine, but does not allow customizations.  It also does not take into
consideration the current line width.

I'm proposing this change:

```diff
@@ In Writers/LaTeX.hs:
-"\\begin{center}\\rule{3in}{0.4pt}\\end{center}"
+"\\begin{center}\\rule{0.5\\linewidth}{\\linethickness}\\end{center}"
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index d200ecee1..966cabe67 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/src/Text/Pandoc/Writers/LaTeX.hs
@@ -465,7 +465,7 @@ blockToLaTeX (DefinitionList lst) = do
   return $ text ("\\begin{description}" ++ inc) $$ spacing $$ vcat items $$
                "\\end{description}"
 blockToLaTeX HorizontalRule = return $
-  "\\begin{center}\\rule{3in}{0.4pt}\\end{center}"
+  "\\begin{center}\\rule{0.5\\linewidth}{\\linethickness}\\end{center}"
 blockToLaTeX (Header level (id',classes,_) lst) =
   sectionHeader ("unnumbered" `elem` classes) id' level lst
 blockToLaTeX (Table caption aligns widths heads rows) = do