support attachments (#88)

src/carbon/attachment.cr

@@ -0,0 +1,7 @@
+module Carbon
+  alias AttachFile = NamedTuple(file_path: String, file_name: String?, mime_type: String?)
+  alias AttachIO = NamedTuple(io: IO, file_name: String, mime_type: String?)
+  alias ResourceFile = NamedTuple(file_path: String, cid: String, file_name: String?, mime_type: String?)
+  alias ResourceIO = NamedTuple(io: IO, cid: String, file_name: String, mime_type: String?)
+  alias Attachment = AttachFile | AttachIO | ResourceFile | ResourceIO
+end

src/carbon/email.cr

@@ -8,7 +8,7 @@ abstract class Carbon::Email
   abstract def from : Carbon::Address
   abstract def to : Array(Carbon::Address)
 
-  def_equals subject, from, to, cc, bcc, headers, text_body, html_body
+  def_equals subject, from, to, cc, bcc, headers, text_body, html_body, attachments
 
   # Set this value to `false` to prevent the email from
   # being delivered
@@ -91,6 +91,18 @@ abstract class Carbon::Email
     end
   end
 
+  @attachments = [] of Carbon::Attachment
+  getter attachments
+
+  macro attachment(value)
+    def attachments : Array(Carbon::Attachment)
+      {% if @type.methods.map(&.name).includes?("attachments".id) %}
+        previous_def
+      {% end %}
+      @attachments << {{value}}
+    end
+  end
+
   {% for method in [:to, :cc, :bcc] %}
     macro {{ method.id }}(value)
       def {{ method.id }} : Array(Carbon::Address)