LaTeX writer: Use a declaration for tight lists

Currently, pandoc has hard-coded the following in order to make tight lists in
LaTeX:

```hs
text "\\itemsep1pt\\parskip0pt\\parsep0pt"
```

Which is fine, but does not allow customizations.  For example, the `memoir`
class already has a `\tightlist` declaration for this purpose:

```tex
\newcommand{\tightlist}{%
  \setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}
```

I'm proposing to use a similar solution:

```diff
@@ In Writers/LaTeX.hs:
-then text "\\itemsep1pt\\parskip0pt\\parsep0pt"
+then text "\\tightlist"

@@ In templates/default.latex:
+\newcommand{\tightlist}{%
+  \setlength{\itemsep}{1pt}\setlength{\parskip}{0pt}\setlength{\parsep}{0pt}}
```

This allows us to customize the tightness to our needs.

Backward Compatibility

If a person is using a custom LaTeX template (not based upon the `memoir`
class), the `\tightlist` declaration must be added.

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/Tests/Writers/LaTeX.hs b/tests/Tests/Writers/LaTeX.hs
index 78b2495c7..965589965 100644
--- a/tests/Tests/Writers/LaTeX.hs
+++ b/tests/Tests/Writers/LaTeX.hs
@@ -42,7 +42,7 @@ tests = [ testGroup "code blocks"
         , testGroup "definition lists"
           [ "with internal link" =: definitionList [(link "#go" "" (str "testing"),
              [plain (text "hi there")])] =?>
-            "\\begin{description}\n\\itemsep1pt\\parskip0pt\\parsep0pt\n\\item[{\\hyperref[go]{testing}}]\nhi there\n\\end{description}"
+            "\\begin{description}\n\\tightlist\n\\item[{\\hyperref[go]{testing}}]\nhi there\n\\end{description}"
           ]
         , testGroup "math"
           [ "escape |" =: para (math "\\sigma|_{\\{x\\}}") =?>