Adding HTML tags ins and del

Github does not support the `<u>` tag for underlined text, in HTML there exists the tag `<ins>`. On https://www.w3schools.com/tags/tag_ins.asp is written:
The `<ins>` tag defines a text that has been inserted into a document.
Browsers will normally strike a line through deleted text and underline inserted text.

Most browsers will display the `<ins>` element with the following default values:
```
ins {
  text-decoration: underline;
}
```

analogous there exists the tag `<del>` (https://www.w3schools.com/tags/tag_del.asp):
The `<del>` tag defines text that has been deleted from a document.
Browsers will normally strike a line through deleted text and underline inserted text.

Most browsers will display the `<del>` element with the following default values:
```
del {
  text-decoration: line-through;
}
```

Definitions analogue to the underline and strike through tag the implementation for the other formats has been chosen.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/printdocvisitor.h b/src/printdocvisitor.h
index 26bb3e7..1472110 100644
--- a/src/printdocvisitor.h
+++ b/src/printdocvisitor.h
@@ -108,9 +108,15 @@ class PrintDocVisitor : public DocVisitor
         case DocStyleChange::Strike:
           if (s->enable()) printf("<strike>"); else printf("</strike>");
           break;
+        case DocStyleChange::Del:
+          if (s->enable()) printf("<del>"); else printf("</del>");
+          break;
         case DocStyleChange::Underline:
           if (s->enable()) printf("<underline>"); else printf("</underline>");
           break;
+        case DocStyleChange::Ins:
+          if (s->enable()) printf("<ins>"); else printf("</ins>");
+          break;
         case DocStyleChange::Italic:
           if (s->enable()) printf("<italic>"); else printf("</italic>");
           break;