Added parsing attachments in non-multipart emails

[ischekoldin]

There are emails with no text in the body that contain only a binary attachment. Mail clients handle those as proper attachments yet parsemail doesn't recognize them.

[Lngramos]

Hello, @ischekoldin - thanks for this, it's exactly what I needed 🙌

@DusanKasan what can be done so this merged in? (thanks for the great library too, by the way)

Aside from the incorrect go.mod file - do we need some test coverage added around this use case? Happy to help towards getting this merged, as I'd much rather use your project as an upstream than using relying on a fork to use these changes.

[Lngramos]

p.s. have also just come across https://github.com/DusanKasan/parsemail/pull/25 which looks like that may also address this use case, indirectly.