Add support for strikethoughs in messages.

Author: Sam1301

app/src/main/java/com/zulip/android/util/CustomHtmlToSpannedConverter.java

@@ -36,6 +36,7 @@
 import android.text.style.ParagraphStyle;
 import android.text.style.QuoteSpan;
 import android.text.style.RelativeSizeSpan;
+import android.text.style.StrikethroughSpan;
 import android.text.style.StyleSpan;
 import android.text.style.SubscriptSpan;
 import android.text.style.SuperscriptSpan;
@@ -525,6 +526,12 @@ private void handleStartTag(String tag, Attributes attributes) {
             start(mSpannableStringBuilder, new InlineCode());
         } else if (tag.equalsIgnoreCase("pre")) {
             start(mSpannableStringBuilder, new CodeBlock());
+        } else if (tag.equalsIgnoreCase("del")) {
+            start(mSpannableStringBuilder, new StrikeThrough());
+        } else if (tag.equalsIgnoreCase("s")) {
+            start(mSpannableStringBuilder, new StrikeThrough());
+        } else if (tag.equalsIgnoreCase("strike")) {
+            start(mSpannableStringBuilder, new StrikeThrough());
         } else if (tag.length() == 2
                 && Character.toLowerCase(tag.charAt(0)) == 'h'
                 && tag.charAt(1) >= '1' && tag.charAt(1) <= '6') {
@@ -599,6 +606,12 @@ private void handleEndTag(String tag) {
         } else if (tag.equalsIgnoreCase("pre")) {
             end(mSpannableStringBuilder, CodeBlock.class, new TypefaceSpan(
                     MONOSPACE));
+        } else if (tag.equalsIgnoreCase("del")) {
+            end(mSpannableStringBuilder, StrikeThrough.class, new StrikethroughSpan());
+        } else if (tag.equalsIgnoreCase("s")) {
+            end(mSpannableStringBuilder, StrikeThrough.class, new StrikethroughSpan());
+        } else if (tag.equalsIgnoreCase("strike")) {
+            end(mSpannableStringBuilder, StrikeThrough.class, new StrikethroughSpan());
         } else if (tag.length() == 2
                 && Character.toLowerCase(tag.charAt(0)) == 'h'
                 && tag.charAt(1) >= '1' && tag.charAt(1) <= '6') {
@@ -718,6 +731,9 @@ private static class InlineCode {
     private static class CodeBlock {
     }
 
+    private static class StrikeThrough {
+    }
+
     private static class Font {
         public String mColor;
         public String mFace;