new header_separator option used to display a separator string between headers and body

This commit adds a customizable separator strings that is displayed between the message headers and body.  Includes theme configuration as well.  By default the separator is None.  The configuration I use (makes it easier on the eyes when visually parsing messages):

```
header_separator = '----'
```

Theme:
```
[global]
    separator = 'default','','light gray','default','dark red','default'
    separator_focus = 'default','','light gray','default','light red','default'
```