Fix argument processor exception from SetCommand

Attempting to do anything involving SetCommand fails for me with e.g.:

```
java.lang.RuntimeException: Can't create argument processor for type class org.cyclopsgroup.jmxterm.cmd.SetCommand with parser org.cyclopsgroup.jcli.GnuParser@5143a680
	at org.cyclopsgroup.jcli.impl.DefaultArgumentProcessorFactory.newProcessor(DefaultArgumentProcessorFactory.java:28)
	at org.cyclopsgroup.jcli.ArgumentProcessor.newInstance(ArgumentProcessor.java:41)
	at org.cyclopsgroup.jcli.ArgumentProcessor.newInstance(ArgumentProcessor.java:28)
	at org.cyclopsgroup.jmxterm.cc.HelpCommand.execute(HelpCommand.java:45)
...
Caused by: java.lang.IllegalArgumentException: Can't find constructor with string for type interface java.util.List
	at org.cyclopsgroup.caff.conversion.SimpleReflectiveConverter.<init>(SimpleReflectiveConverter.java:39)
	at org.cyclopsgroup.caff.conversion.SimpleConverter.<init>(SimpleConverter.java:84)
	at org.cyclopsgroup.caff.conversion.AnnotatedConverter$Builder.toConverter(AnnotatedConverter.java:25)
	at org.cyclopsgroup.caff.conversion.AnnotatedConverter$Builder.access$200(AnnotatedConverter.java:16)
	at org.cyclopsgroup.caff.conversion.AnnotatedConverter.<init>(AnnotatedConverter.java:108)
	at org.cyclopsgroup.jcli.impl.ParsingContextBuilder.createReference(ParsingContextBuilder.java:41)
	at org.cyclopsgroup.jcli.impl.ParsingContextBuilder.build(ParsingContextBuilder.java:115)
	at org.cyclopsgroup.jcli.impl.DefaultArgumentProcessor.<init>(DefaultArgumentProcessor.java:24)
	at org.cyclopsgroup.jcli.impl.DefaultArgumentProcessorFactory.newProcessor(DefaultArgumentProcessorFactory.java:24)
	... 22 more
Caused by: java.lang.NoSuchMethodException: java.util.List.<init>(java.lang.String)
	at java.lang.Class.getConstructor0(Class.java:2800)
	at java.lang.Class.getConstructor(Class.java:1708)
	at org.cyclopsgroup.caff.conversion.SimpleReflectiveConverter.<init>(SimpleReflectiveConverter.java:30)
	... 30 more
```

Annotating with @MultiValue appears to fix it.