Adds color mappings to Log4net appender

Adds color mappings to log4net appender:

```xml
<mapping>
  <level value="Debug" />
  <foreColor value="Black" />
  <backColor value="Red" />
  <attribute value="Bold" />
  <attribute value="Underline" />
</mapping>
```

Will result in a red line with black letters underlined and bold when the line is logged with level debug.

@SurajGupta do you have enough experience with NLog to add this to the NLog target.