Note about non litteral in annotations

[alexarchambault]

Annotations (@ExtraName, @AppName, @AppVersion, ...) expect litteral arguments. They'll likely crash with variable arguments.

Add a note about that in doc.

[alexarchambault]

About @AppVersion, it could be practical to be able to provide its value at runtime, to be able to use the values provided by sbt-buildinfo for example (although these could type their values with singleton types to circumvent that!).

AppOf should have an (overridable) method like

def appVersion: String

where the version for the help message would be read. By default, it would simply provide the version from the @AppVersion annotation.