Add API to facilitate tabular custom Help sections

[remkop]

Related: #856

Currently, if a user wants to show some data (for example environment variables) in tabular format, they need to implement IHelpSectionRenderer and have custom logic that builds a TextTable.

Picocli internally has similar logic to build a TextTable for the exit codes section. It makes sense to expose some of the mechanics for this as public API to reduce custom code in client applications.

Proposed new methods:

• Help.createHeading(String, Object...)
• Help.createTextTable(Map<?, ?>)

Example usage:

// help section keys
static final String SECTION_KEY_ENV_HEADING = "environmentVariablesHeading";
static final String SECTION_KEY_ENV_DETAILS = "environmentVariables";
// ...

// the data to display
Map<String, String> env = new LinkedHashMap<>();
env.put("FOO", "explanation of foo");
env.put("BAR", "explanation of bar");
env.put("XYZ", "xxxx yyyy zzz");

// register the custom section renderers
CommandLine cmd = new CommandLine(new MyApp());
cmd.getHelpSectionMap().put(SECTION_KEY_ENV_HEADING,
                            help -> help.createHeading("Environment Variables:%n"));
cmd.getHelpSectionMap().put(SECTION_KEY_ENV_DETAILS,
                            help -> help.createTextTable(env).toString());

// specify the location of the new sections
List<String> keys = new ArrayList<>(cmd.getHelpSectionKeys());
int index = keys.indexOf(CommandLine.Model.UsageMessageSpec.SECTION_KEY_FOOTER_HEADING);
keys.add(index, SECTION_KEY_ENV_HEADING);
keys.add(index + 1, SECTION_KEY_ENV_DETAILS);
cmd.setHelpSectionKeys(keys);