Don't print out excessive ANSI codes

The ANSI renderer was printing out astonishing quantities of SGR 0s,
which don't hurt anything but has bad code aesthetics. Fix it to only
print out font requests when the font changes.

before:

```
♪ pandoc$ cabal exec -- pandoc -t ansi doc/customizing-pandoc.md  | wc -c
   17986
```

after:

```
♪ pandoc$ cabal exec -- pandoc -t ansi doc/customizing-pandoc.md  | wc -c
    5297
```

src/Text/DocLayout.hs

@@ -289,8 +289,8 @@ render :: HasChars a => Maybe Int -> Doc a -> a
 render = renderPlain
 
 renderANSI :: HasChars a => Maybe Int -> Doc a -> a
-renderANSI n d = go $ prerender n d where
-  go (Attributed s) = foldMap attrRender s
+renderANSI n d = snd $ go $ prerender n d where
+  go (Attributed s) = foldl attrRender (baseFont, "") s
 
 renderPlain :: HasChars a => Maybe Int -> Doc a -> a
 renderPlain n d = go $ prerender n d where
@@ -300,10 +300,11 @@ attrStrip :: HasChars a => Attr a -> a
 attrStrip (Attr _ y) | isNull y = ""
                      | otherwise = y
 
-attrRender :: HasChars a => Attr a -> a
-attrRender a = go a <> renderFont baseFont where
-  go (Attr f y) | isNull y = ""
-                | otherwise = renderFont f <> y
+attrRender :: HasChars a => (Font, a) -> Attr a -> (Font, a)
+attrRender (f, acc) (Attr g y)
+  | isNull y = (f, acc)
+  | f == g = (f, acc <> y)
+  | otherwise = (g, acc <> renderFont g <> y)
 
 prerender :: HasChars a => Maybe Int -> Doc a -> Attributed a
 prerender linelen doc = fromList . reverse . output $