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/htmldocvisitor.cpp b/src/htmldocvisitor.cpp
index 37f6bd0..6e637f2 100644
--- a/src/htmldocvisitor.cpp
+++ b/src/htmldocvisitor.cpp
@@ -403,9 +403,15 @@ void HtmlDocVisitor::visit(DocStyleChange *s)
     case DocStyleChange::Strike:
       if (s->enable()) m_t << "<strike" << htmlAttribsToString(s->attribs()) << ">";      else m_t << "</strike>";
       break;
+    case DocStyleChange::Del:
+      if (s->enable()) m_t << "<del" << htmlAttribsToString(s->attribs()) << ">";      else m_t << "</del>";
+      break;
     case DocStyleChange::Underline:
       if (s->enable()) m_t << "<u" << htmlAttribsToString(s->attribs()) << ">";      else m_t << "</u>";
       break;
+    case DocStyleChange::Ins:
+      if (s->enable()) m_t << "<ins" << htmlAttribsToString(s->attribs()) << ">";      else m_t << "</ins>";
+      break;
     case DocStyleChange::Italic:
       if (s->enable()) m_t << "<em" << htmlAttribsToString(s->attribs()) << ">";     else m_t << "</em>";
       break;