LaTeX reader: Support simple `\def` macros.

Note that we still don't support macros with fancy parameter
delimiters, like

    \def\foo#1..#2{...}

1 files changed, 28 insertions, 0 deletions

diff --git a/test/command/macros.md b/test/command/macros.md
index 49a648c79..46179e3c7 100644
--- a/test/command/macros.md
+++ b/test/command/macros.md
@@ -38,3 +38,31 @@ expanded at point of use:
 \emph{ouk}
 ```
 
+```
+% pandoc -f latex -t latex
+\def\BDpos{}
+\def\BDneg{-}
+\def\beq{\begin{align}}
+\def\eeq{\end{align}}
+\def\e#1{\emph{#1}}
+\def\f#1#2{\emph{#1--#2}}
+
+$5\BDneg 6\BDpos 7$
+
+\beq
+x &= y\\
+\eeq
+
+\e{hi}
+
+\f{hi}{ok}
+^D
+\(5-67\)
+
+\[\begin{aligned}
+x &= y\\\end{aligned}\]
+
+\emph{hi}
+
+\emph{hi--ok}
+```