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, 3 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index 8e3befe19..0fa1e4857 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/src/Text/Pandoc/Writers/LaTeX.hs
@@ -411,7 +411,7 @@ blockToLaTeX (BulletList lst) = do
   let inc = if incremental then "[<+->]" else ""
   items <- mapM listItemToLaTeX lst
   let spacing = if isTightList lst
-                   then text "\\itemsep1pt\\parskip0pt\\parsep0pt"
+                   then text "\\tightlist"
                    else empty
   return $ text ("\\begin{itemize}" ++ inc) $$ spacing $$ vcat items $$
              "\\end{itemize}"
@@ -446,7 +446,7 @@ blockToLaTeX (OrderedList (start, numstyle, numdelim) lst) = do
                         else "\\setcounter" <> braces enum <>
                               braces (text $ show $ start - 1)
   let spacing = if isTightList lst
-                   then text "\\itemsep1pt\\parskip0pt\\parsep0pt"
+                   then text "\\tightlist"
                    else empty
   return $ text ("\\begin{enumerate}" ++ inc)
          $$ stylecommand
@@ -460,7 +460,7 @@ blockToLaTeX (DefinitionList lst) = do
   let inc = if incremental then "[<+->]" else ""
   items <- mapM defListItemToLaTeX lst
   let spacing = if all isTightList (map snd lst)
-                   then text "\\itemsep1pt\\parskip0pt\\parsep0pt"
+                   then text "\\tightlist"
                    else empty
   return $ text ("\\begin{description}" ++ inc) $$ spacing $$ vcat items $$
                "\\end{description}"