vimhelp: Add custom MacVim filter to show a nice header at gui_mac.txt

Add custom logic to locate the "MACVIM REFERENCE MANUAL" and replace
with a logo and header. This is a somewhat hacky solution rather than a
principled markup based method, but it works. No need to overcomplicate
things.

Author: ychin

vimhelp/static/MacVim.png

@@ -280,6 +280,12 @@ def to_html(self, filename, contents):
             )
             lastpos = 0
 
+            # Custom MacVim filters
+            if filename == "gui_mac.txt":
+                if re.fullmatch("\s*MACVIM REFERENCE MANUAL\s*", line) != None:
+                    out.append("<h1><img src=MacVim.png width=64px>\nMacVim Reference Manual</h1>")
+                    line = ""
+
             tab_fixer = TabFixer()
 
             for match in RE_TAGWORD.finditer(line):