Support arbitrary element names.

Standard HTML elements can now be defined using a combination of `frag`
and `el` as shown in the example `.slab` file.

examples/el.html

@@ -0,0 +1,20 @@
+<x>
+    Hello.
+</x>
+<x>
+    <p>
+        Hello.
+    </p>
+</x>
+<y>
+    Hello.
+</y>
+<y>
+    <p>
+        Hello.
+    </p>
+</y>
+<y>
+</y>
+<z class="a">
+</z>

examples/el.slab

@@ -0,0 +1,17 @@
+el x Hello.
+
+el x
+  p Hello.
+
+frag y
+  el y
+    default content
+
+y Hello.
+
+y
+  p Hello.
+
+y
+
+el z.a

src/Slab/Parse.hs

@@ -266,7 +266,9 @@ parserNameWithAttrs :: Parser (Elem, [Attr], TrailingSym)
 parserNameWithAttrs =
   lexeme
     ( do
-        a <- parserElem
+        a <-
+          parserElem
+            <|> (lexeme (string "el") *> (Elem <$> lexeme parserName))
         -- `try` because we want to backtrack if there is a dot
         -- not followed by a class name, for mdot to succeed.
         b <- many parserAttrs'

src/Slab/Render.hs

@@ -14,6 +14,7 @@ import Text.Blaze.Html.Renderer.Text (renderHtml)
 import Text.Blaze.Html5 (Html, (!))
 import Text.Blaze.Html5 qualified as H
 import Text.Blaze.Html5.Attributes qualified as A
+import Text.Blaze.Internal qualified as I
 import Text.Blaze.Svg11 qualified as S
 
 --------------------------------------------------------------------------------
@@ -195,3 +196,4 @@ renderElem = \case
   Syntax.Svg -> S.svg
   Syntax.Textarea -> H.textarea
   Syntax.Canvas -> H.canvas
+  Syntax.Elem name -> I.customParent (H.textTag name)

src/Slab/Syntax.hs

@@ -143,6 +143,8 @@ data Elem
   | Svg
   | Textarea
   | Canvas
+  | -- | Arbitrary element name, using the @el@ keyword.
+    Elem Text
   deriving (Show, Eq)
 
 data TrailingSym = HasDot | HasEqual | NoSym