Add siunitx Support (#3588)

For example:


```latex
\SI[round-precision=2]{1}{m} is equal to \SI{1000}{mm}.

\SI[round-precision=2]{1}[\$]{} is equal to \SI{0.938094}{\euro}
```

2 files changed, 38 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Readers/LaTeX.hs b/src/Text/Pandoc/Readers/LaTeX.hs
index e85002ba3..f3c94dacb 100644
--- a/src/Text/Pandoc/Readers/LaTeX.hs
+++ b/src/Text/Pandoc/Readers/LaTeX.hs
@@ -668,6 +668,8 @@ inlineCommands = M.fromList $
   , ("nocite", mempty <$ (citation "nocite" NormalCitation False >>=
                           addMeta "nocite"))
   , ("hypertarget", braced >> tok)
+  -- siuntix
+  , ("SI", dosiunitx)
   ] ++ map ignoreInlines
   -- these commands will be ignored unless --parse-raw is specified,
   -- in which case they will appear as raw latex blocks:
@@ -726,6 +728,21 @@ dolstinline = do
 doLHSverb :: PandocMonad m => LP m Inlines
 doLHSverb = codeWith ("",["haskell"],[]) <$> manyTill (satisfy (/='\n')) (char '|')
 
+-- converts e.g. \SI{1}[\$]{} to "$ 1" or \SI{1}{\euro} to "1 €"
+dosiunitx :: PandocMonad m => LP m Inlines
+dosiunitx = do
+  skipopts
+  value <- tok
+  valueprefix <- option "" $ char '[' >> (mconcat <$> manyTill tok (char ']'))
+  unit <- tok
+  let emptyOr160 "" = ""
+      emptyOr160 _  = "\160"
+  return . mconcat $ [valueprefix, 
+                      emptyOr160 valueprefix,
+                      value, 
+                      emptyOr160 unit,
+                      unit]
+
 lit :: String -> LP m Inlines
 lit = pure . str
 
diff --git a/test/command/3587.md b/test/command/3587.md
new file mode 100644
index 000000000..addb6c582
--- /dev/null
+++ b/test/command/3587.md
@@ -0,0 +1,21 @@
+```
+% pandoc -f latex -t native
+\SI[round-precision=2]{1}{m} is equal to \SI{1000}{mm}
+^D
+[Para [Str "1\160m",Space,Str "is",Space,Str "equal",Space,Str "to",Space,Str "1000\160mm"]]
+```
+
+```
+% pandoc -f latex -t native
+\SI[round-precision=2]{1}[\$]{} is equal to \SI{0.938094}{\euro}
+^D
+[Para [Str "$\160\&1",Space,Str "is",Space,Str "equal",Space,Str "to",Space,Str "0.938094\160\8364"]]
+```
+
+
+```
+% pandoc -f latex -t native
+\SI[round-precision=2]{\{\}}[\{\}]{\{\}}
+^D
+[Para [Str "{}\160{}\160{}"]]
+```