Make INCBIN's length argument optional

INCBIN can now be used with just a start position to include everything
from the start position until the end of the file.

docs/rgbasm.5.html

@@ -1346,6 +1346,9 @@ INCBIN "sprites/hero.bin"
 INCBIN "data.bin",78,256
 </pre>
 </div>
+<p class="Pp">The length arugment is optional. If only the start position is
+    specified, the bytes from the start position until the end of the file will
+    be included.</p>
 </section>
 <section class="Ss">
 <h2 class="Ss" id="Unions"><a class="permalink" href="#Unions">Unions</a></h2>