Markdown.pl: make the default yaml API mode match CLI

The default YAML mode from the command line shows unknown
YAML options in a table prefixed to the output and applies
the ones it recognizes.

Make the API have the same default mode rather than silently
discarding unknown YAML options and ignoring known ones.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -765,6 +765,7 @@ sub ProcessRaw {
 #               if < 0 any YAML front matter will be parsed but no
 #               options will be applied at all.  When != 0 the parsed
 #               YAML front matter can be retrieved via the 'yaml' key.
+#               defaults to 1 if not defined or not a number.
 #   yamlvis    => 0 (invisible), > 0 (visible), < 0 (vis if unknown)
 #               if yamlmode == 0 then yamlvis has no effect.  if > 0
 #               then any parsed YAML front matter options will be shown
@@ -773,6 +774,7 @@ sub ProcessRaw {
 #               if < 0 then YAML front matter options will be shown in
 #               the formatted output only if there are any unrecognized
 #               options present.
+#               defaults to -1 if not defined or not a number.
 #   keepabs    => any-false-value (no action), any-true-value (keep)
 #               if true, any absolute path URLs remaining after applying
 #               any abs_prefix value will be kept and not be subject
@@ -925,10 +927,10 @@ sub _SanitizeOpts {
     # already exists).
     delete $o->{h1};
 
-    # Default is to silently strip any YAML front matter
+    # Default is to silently strip any known YAML front matter
     # Same comment about "yaml" key as above for "h1" key
-    $o->{yamlmode} = -1 unless looks_like_number($o->{yamlmode});
+    $o->{yamlmode} = 1 unless looks_like_number($o->{yamlmode});
-    $o->{yamlvis} = 0 unless looks_like_number($o->{yamlvis});
+    $o->{yamlvis} = -1 unless looks_like_number($o->{yamlvis});
     delete $o->{yaml};
 
     # The anchors hash will only be returned if the key exists