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, 3 insertions, 1 deletions

diff --git a/src/docparser.h b/src/docparser.h
index 15180f9..8e3b266 100644
--- a/src/docparser.h
+++ b/src/docparser.h
@@ -382,7 +382,9 @@ class DocStyleChange : public DocNode
                  Span          = (1<<8),
                  Div           = (1<<9),
                  Strike        = (1<<10),
-                 Underline     = (1<<11)
+                 Underline     = (1<<11),
+                 Del           = (1<<12),
+                 Ins           = (1<<13)
                };
 
     DocStyleChange(DocNode *parent,uint position,Style s,bool enable,