add a VERSION constant (#471)

spec/config_spec.cr

@@ -54,4 +54,12 @@ describe "Config" do
     Kemal::CLI.new
     test_option.should eq("FOOBAR")
   end
+
+  it "returns a string" do
+    Kemal::VERSION.should be_a(String)
+  end
+
+  it "gets the version from shards.yml" do
+    Kemal::VERSION.should eq("0.23.0")
+  end
 end

src/kemal/config.cr

@@ -1,4 +1,6 @@
 module Kemal
+  VERSION = {{ `shards version #{__DIR__}`.chomp.stringify }}
+
   # Stores all the configuration options for a Kemal application.
   # It's a singleton and you can access it like.
   #