Add shortcut method for creating environments - close #149

Add env() method to create environments.
Process all environments created at build script top level before
executing any autoconf-enabled tasks, or if no tasks are specified by
the user.

build_tests/d/Rsconscript

@@ -2,8 +2,6 @@ configure do
   check_d_compiler "gdc"
 end
 
-default do
-  Environment.new(echo: :command) do |env|
-    env.Program("hello-d.exe", glob("*.d"))
-  end
+env(echo: :command) do |env|
+  env.Program("hello-d.exe", glob("*.d"))
 end