Add of archiveOldFileOnStartup parameter in FileTarget

This adds the possibility of archiving an existing log file every time an application starts. As described/proposed in #213.
The added parameter for the File target is <code>archiveOldFileOnStartup</code>.

The following config will archive the current log file each time the application is started.
```xml
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <targets>
    <target name="filelog"
            xsi:type="File"
            FileName="${basedir}/log.txt"
            layout="${longdate} ${message}"
            archiveFileName="${basedir}/Archives/log.{#}.txt"
            archiveNumbering="Rolling"
            archiveOldFileOnStartup="true"/>
  </targets>
  <rules>
    <logger name="Logger1" writeTo="filelog"></logger>
  </rules>
</nlog>
```
(The xsd and wiki needs to be updated if this PR is accepted)