Fixes passing --help to subcommand

Avoid OptionParser to short-circuit `--help` and return immediately by
setting a flag for it and evaluating at the end of the processing of
unknown options.

This is only done for the CLI invocation and is not part of Shards
module (as the help and usage options are only available in this
context).

spec/integration/subcommand_spec.cr

@@ -10,6 +10,16 @@ describe "subcommand" do
       output.should contain(%(args: ["--no-color", "--verbose", "--unknown", "other", "argument"]))
     end
   end
+
+  it "correctly forwards '--help' option to subcommand" do
+    create_shard("dummy", "0.1.0")
+    create_executable "dummy", "bin/shards-dummy", %(print "args: "; print ARGV)
+
+    with_path(git_path("dummy/bin")) do
+      output = run("shards dummy --help")
+      output.should contain(%(args: ["--help"]))
+    end
+  end
 end
 
 private def with_path(path)

src/cli.cr

@@ -2,6 +2,8 @@ require "option_parser"
 require "./commands/*"
 
 module Shards
+  class_property? display_help : Bool = false
+
   def self.display_help_and_exit(opts)
     puts <<-HELP
       shards [<options>...] [<command>]
@@ -53,7 +55,7 @@ module Shards
       opts.on("--ignore-crystal-version", "Has no effect. Kept for compatibility, to be removed in the future.") { }
       opts.on("-v", "--verbose", "Increase the log verbosity, printing all debug statements.") { self.set_debug_log_level }
       opts.on("-q", "--quiet", "Decrease the log verbosity, printing only warnings and errors.") { self.set_warning_log_level }
-      opts.on("-h", "--help", "Print usage synopsis.") { self.display_help_and_exit(opts) }
+      opts.on("-h", "--help", "Print usage synopsis.") { self.display_help = true }
 
       opts.unknown_args do |args, options|
         case args[0]? || DEFAULT_COMMAND
@@ -107,6 +109,10 @@ module Shards
           end
         end
 
+        if display_help?
+          display_help_and_exit(opts)
+        end
+
         exit
       end
     end