Add support for StartSelection EndSelection headers

Today I faced following structure:

```
Version:0.9
StartHTML:000000149
EndHTML:000003971
StartFragment:000001422
EndFragment:000003939
StartSelection:000001422
EndSelection:000003939
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html><head><style>ul (...)
```

Produced by eM Client. It makes sense to support these extra headers.