Add the ability to display expanded configurations

[julen]

When running serge <command> <config-file>.serge, the configuration file might be inheriting key-values from some common or master configuration file, and one might need to browse files manually to realize of the entire set of options in effect.

While a standalone Config::Neat command already supports displaying the expanded/interpreted configurations, it probably makes sense to integrate/wrap it in a simple serge command such as serge show <config-file>.serge. Bonus points for coloring the syntax in the console and running that through a pager.

[iafan]

While I merged the PR and the show command is now available, I'll keep the issue open for two enhancements mentioned:

• syntax highlighting (depends on Add an ability to render the config with colorizing iafan/Config-Neat#6)
• pagination support (similar to what serge help already does)