Add extension mechanism

Some alias duplicate an existing alias by extending the command. In order to reduce redundance a simple templating mechanism should be provided.

The addition to the XML file may look like this:

<extension>
<name>aq</name>
<template>@echo off

{@cmd} @echo on</template>
<comment>xyz</comment>
<apply-to>
<group>mvn</group>
<group>project-release</group>
<alias>r</alias>
</apply-to>
</extension>

The aliases mentioned in the 'apply-to' are applied to the template (the original aliases will still be created). The alias 'r' in the example will be created in addition to the alias 'raq'. The place holder {@cmd}

will contain the extended command.