CommandPromptPortable: Target directory as parameter

In order to be able to use CommandPromptPortable for launching command prompts from within portable 3rd-party Explorer replacements (like FreeCommander) directly into the current working directory, I was slightly changing the CommandPromptPortable launcher source (CommandPromptPortable.nsi). It reads the target directory as parameter now, e.g. running

P:\PortableApps\CommandPromptPortable>CommandPromptPortable.exe X:\foo\bar

would result in X:\foo\bar being the current directory of the command prompt being launched, like

X:\foo\bar>

Here's what I changed:

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 		IfFileExists "$EXEDIR\Data\Batch\commandprompt.bat" "" LaunchNow
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