Add cat macro

This concatenates all the arguments without spaces.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

markup.py

@@ -246,6 +246,11 @@ def macro_round(ctx: MarkupContext, n: str, digits: Optional[str] = None) -> str
     return str(res)
 
 
+def macro_cat(ctx: MarkupContext, *text: str) -> str:
+    "Concatenates all arguments."
+    return ''.join(text)
+
+
 @functools.cache
 def default_context():
     c = MarkupContext()
@@ -253,6 +258,8 @@ def default_context():
     c.register_macro("choose", macro_choose)
     c.register_macro("round", macro_round)
 
+    c.register_macro("cat", macro_cat)
+
     return c