Added an option to manually set filename of attachment

[slavikm]

If the name of the attachment is different the the actual filename on disk, we should have an easy option to change it

[slavikm]

@alexcesaro sorry about the noise with the 2 PRs. I did not see that my teammate already created a PR on the exact same issue.

[alexcesaro]

Please limit the line length to 80 characters and add a dot at the end of the sentence.

I think Rename() might be more clear. SetFilename can be confusing, some people could think you have to put the filename on disk here.

[alexcesaro]

Thanks, I commented the code.

Please also merge all your commits (I only want one commit at the end) and add an example in example_test.go.

[slavikm]

@alexcesaro Not sure how the examples are being used but see the last commit - squashed and added example.

[alexcesaro]

Thank you! I slightly edited your commit to fix the example.

For your information, rules about naming examples can be found in the documentation of the testing package. And you can see the result in the Gomail documentation.