[Specs] Adds convenience method context

Context does the same as describe, but is more semantic on some contexts
(no pun intended).

spec/std/spec_spec.cr

@@ -40,4 +40,10 @@ describe "Spec matchers" do
       42.should_not be_falsey
     end
   end
+
+  context "should work as describe" do
+    it "is true" do
+      true.should be_truthy
+    end
+  end
 end

src/spec/spec.cr

@@ -82,6 +82,10 @@ def describe(description, file = __FILE__, line = __LINE__)
   end
 end
 
+def context(description, file = __FILE__, line = __LINE__, &block)
+  describe(description.to_s, file, line, &block)
+end
+
 def it(description, file = __FILE__, line = __LINE__)
   return if Spec.aborted?
   return unless Spec.matches?(description, file, line)