Includes debug flag when compiling in debug mode

Allow usage of `flag?(:debug)` in macros for builds been done with
`--debug` option.

This pairs with `--release` and `flag?(:release)` usage in similar
way to allow include or exclude code from the compilation process.

    {% if flag?(:debug) %}
      # debug-only code
    {% end %}

src/compiler/crystal/compiler.cr

@@ -151,7 +151,8 @@ module Crystal
       program = Program.new
       program.cache_dir = CacheDir.instance.directory_for(sources)
       program.target_machine = target_machine
-      program.flags << "release" if @release
+      program.flags << "release" if release?
+      program.flags << "debug" if debug?
       program.flags.merge! @flags
       program.wants_doc = wants_doc?
       program.color = color?