Add "drop" grammar keyword to drop patterns

lib/imbecile/grammar.rb

@@ -40,6 +40,9 @@ module Imbecile
           end
           @tokens << Token.new(name, pattern, @tokens.size)
           token_names << name
+        elsif line =~ /^\s*drop\s+(\S+)$/
+          pattern = $1
+          @tokens << Token.new(nil, pattern, @tokens.size)
         else
           raise Error.new("Unexpected input on line #{line_number}: #{line}")
         end

spec/imbecile_spec.rb

@@ -103,6 +103,19 @@ EOF
 token foo
 token bar
 token WS \\s+
+EOF
+  end
+
+  it "allows dropping a matched pattern" do
+    expected = [
+      ["foo", "foo"],
+      [nil, " \t"],
+      ["bar", "bar"],
+    ]
+    expect(run(<<EOF, "foo \tbar")).to eq expected
+token foo
+token bar
+drop \\s+
 EOF
   end
 end