Switches to neato only in some cases.

Check [this
discussion](https://irclog.perlgeek.de/perl6/2018-03-23#i_15958129)
for background.

(Hopefully) closes #246

lib/Perl6/TypeGraph/Viz.pm6

@@ -101,7 +101,9 @@ class Perl6::TypeGraph::Viz {
             run 'dot', '-V', :!err or die 'dot command failed! (did you install Graphviz?)';
         }
         die "bad filename '$file'" unless $file;
-        my $dot = Proc::Async.new(:w, 'dot', '-T', $format, '-o', $file);
+	my $graphvizzer = ( $file ~~ /Metamodel\:\:/ )??'neato'!!'dot';
+	say "$graphvizzer for $file";
+        my $dot = Proc::Async.new(:w, $graphvizzer, '-T', $format, '-o', $file);
         my $promise = $dot.start;
         await($dot.write(self.as-dot(:$size).encode));
         $dot.close-stdin;