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}"
```
Merge pull request #1574 from jlduran/latex-horizontal-rule

LaTeX writer: Make Horizontal Rules more flexible