Optionally use sassc

ruby's sass is officially abandoned, and it has problems with some css files,
for example the latest bootstrap css, so it would be good to have the option
to use sassc. Or even other sass implementations.

This PR will use sassc if `MOJO_PLUGIN_ASSETPACK_SASSC` is set.

lib/Mojolicious/Plugin/AssetPack/Pipe/Sass.pm

@@ -61,9 +61,15 @@ sub process {
       $asset->content($store->save(\$css, $attrs))->FROM_JSON($attrs);
     }
     else {
-      my @args = (qw(sass -s), map { ('-I', $_) } @{$opts{include_paths}});
-      push @args, '--scss'          if $asset->format eq 'scss';
-      push @args, qw(-t compressed) if $attrs->{minified};
+      my @args;
+      if ($ENV{MOJO_PLUGIN_ASSETPACK_SASSC}) {
+        @args = (qw(sassc -s), map { ('-I', $_) } @{$opts{include_paths}});
+      }
+      else {
+        @args = (qw(sass -s --trace), map { ('-I', $_) } @{$opts{include_paths}});
+        push @args, '--scss'          if $asset->format eq 'scss';
+        push @args, qw(-t compressed) if $attrs->{minified};
+      }
       $self->run(\@args, \$content, \my $css, undef);
       $asset->content($store->save(\$css, $attrs))->FROM_JSON($attrs);
     }