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, 2 insertions, 0 deletions

diff --git a/src/rtfdocvisitor.cpp b/src/rtfdocvisitor.cpp
index 55c03a5..efeac64 100644
--- a/src/rtfdocvisitor.cpp
+++ b/src/rtfdocvisitor.cpp
@@ -236,9 +236,11 @@ void RTFDocVisitor::visit(DocStyleChange *s)
       if (s->enable()) m_t << "{\\b ";      else m_t << "} ";
       break;
     case DocStyleChange::Strike:
+    case DocStyleChange::Del:
       if (s->enable()) m_t << "{\\strike ";      else m_t << "} ";
       break;
     case DocStyleChange::Underline:
+    case DocStyleChange::Ins:
       if (s->enable()) m_t << "{\\ul ";      else m_t << "} ";
       break;
     case DocStyleChange::Italic: