Fully honor no color option when running specs

Spec runner have the option to disable colored output (ANSI codes) when
`--no-color` option is used, example:

    $ crystal spec -- --no-color

However, certain elements in the output didn't fully support that option,
resulting in a mix of non-color and color.

This change unifies that setting and ensures that `--no-color` usage
is fully honored by the output.

Ref #4292

src/spec/context.cr

@@ -95,7 +95,7 @@ module Spec
 
             if ex.is_a?(AssertionFailed)
               puts
-              puts "     # #{Spec.relative_file(ex.file)}:#{ex.line}".colorize.cyan
+              puts Spec.color("     # #{Spec.relative_file(ex.file)}:#{ex.line}", :comment)
             end
           end
         end
@@ -137,8 +137,8 @@ module Spec
         puts "Failed examples:"
         puts
         failures_and_errors.each do |fail|
-          print "crystal spec #{Spec.relative_file(fail.file)}:#{fail.line}".colorize.red
-          puts " # #{fail.description}".colorize.cyan
+          print Spec.color("crystal spec #{Spec.relative_file(fail.file)}:#{fail.line}", :error)
+          puts Spec.color(" # #{fail.description}", :comment)
         end
       end
     end

src/spec/dsl.cr

@@ -8,6 +8,7 @@ module Spec
     fail:    :red,
     error:   :red,
     pending: :yellow,
+    comment: :cyan,
   }
 
   private LETTERS = {