Paragraph text can now explicitly be spanned

Background:
Consumers of the library may for instance want line spacing for the
paragraph text but TextView's `lineSpacingMultiplier` and/or
`lineSpacingExtra` apply line spacing to all lines.

With this change, paragraphs may explicitly be spanned with a paragraph
affecting span such as `LineHeightSpan` while the other tags
(e.g. headings) will be unaffected.

library/src/main/java/ru/noties/markwon/SpannableFactory.java

@@ -56,6 +56,9 @@ public interface SpannableFactory {
             boolean isHeader,
             boolean isOdd);
 
+    @Nullable
+    Object paragraph(boolean inTightList);
+
     @Nullable
     Object image(
             @NonNull SpannableTheme theme,

library/src/main/java/ru/noties/markwon/SpannableFactoryDef.java

@@ -103,6 +103,12 @@ public class SpannableFactoryDef implements SpannableFactory {
         return new TableRowSpan(theme, cells, isHeader, isOdd);
     }
 
+    @Nullable
+    @Override
+    public Object paragraph(boolean inTightList) {
+        return null;
+    }
+
     @Nullable
     @Override
     public Object image(@NonNull SpannableTheme theme, @NonNull String destination, @NonNull AsyncDrawable.Loader loader, @NonNull ImageSizeResolver imageSizeResolver, @Nullable ImageSize imageSize, boolean replacementTextIsLink) {

library/src/main/java/ru/noties/markwon/renderer/SpannableMarkdownVisitor.java

@@ -380,14 +380,15 @@ public class SpannableMarkdownVisitor extends AbstractVisitor {
 
     @Override
     public void visit(Paragraph paragraph) {
-
         final boolean inTightList = isInTightList(paragraph);
 
         if (!inTightList) {
             newLine();
         }
 
+        final int length = builder.length();
         visitChildren(paragraph);
+        setSpan(length, factory.paragraph(inTightList));
 
         if (!inTightList) {
             newLine();