#275 fix outdated javadoc for Option#usageHelp and #versionHelp; add method `addSubcommand(String, CommandSpec)`
#275 fix outdated javadoc for Option#usageHelp and #versionHelp; add method `addSubcommand(String, CommandSpec)`

Closes #275

remkop/picocli,remkop/picocli,remkop/picocli,remkop/picocli