Support for required options

This commit introduces support for required options,
which are options that cause the parser to raise
a `MissingRequiredOption` exception if not present.

Options can be marked as required by passing
`required: true` in their configuration, and any errors
caused by missing required options can be suppressed via
`suppress_errors: true`.

lib/slop/error.rb

@@ -32,4 +32,9 @@ def initialize(msg, flag)
       @flag = flag
     end
   end
+
+  # Raised when a required option is *not* parsed.
+  # Suppress with the `suppress_errors` config option.
+  class MissingRequiredOption < Error
+  end
 end

lib/slop/option.rb

@@ -4,6 +4,7 @@ class Option
       help: true,
       tail: false,
       underscore_flags: true,
+      required: false,
     }
 
     # An Array of flags this option matches.

lib/slop/parser.rb

@@ -80,6 +80,13 @@ def parse(strings)
 
       @arguments += ignored_args
 
+      unused_options.each do |o|
+        if o.config[:required] && !suppress_errors?
+          pretty_flags = o.flags.map { |f| "`#{f}'" }.join(", ")
+          raise MissingRequiredOption, "missing required option #{pretty_flags}"
+        end
+      end
+
       Result.new(self).tap do |result|
         used_options.each { |o| o.finish(result) }
       end

test/error_test.rb

@@ -49,3 +49,17 @@
     opts.parse %w(--foo)
   end
 end
+
+describe Slop::MissingRequiredOption do
+  it "raises when a required option is missing" do
+    opts = Slop::Options.new
+    opts.string "-n", "--name", required: true
+    assert_raises(Slop::MissingRequiredOption) { opts.parse [] }
+  end
+
+  it "does not raise when errors are suppressed" do
+    opts = Slop::Options.new(suppress_errors: true)
+    opts.string "-n", "--name", required: true
+    opts.parse []
+  end
+end