Add the Content-Disposition header to attachments

[BalmungSan]

Related to #24

I guess since the job is already done it doesn't hurt to add this, but if you prefer not then I have no objections.

[minosiants]

In this spec https://tools.ietf.org/html/rfc2183 you can see this

Content-Disposition is an optional header field. In its absence, the
   MUA may use whatever presentation method it deems suitable.

If add this it should be made as an optional but it will add complexity to email creation. I wanted to remove all headers definitions from users

[BalmungSan]

@minosiants as you can see, the header is optional and for now is only used when adding an attachment. I think it can be left like that for now and expanded latter if needed.
Or, if you prefer, we can just close this.

[minosiants]

We can keep it open and if we see that it is useful I will merge.
Thanks a lot for your contribution 👍 . I think your are THE FIRST who is using this lib

[BalmungSan]

I think your are THE FIRST who is using this lib

Well I am using it because someone recommended it to me, so no 😉