feat: encoding options added

[pgradoboev]

James, please look at the proposed changes. #358

[coveralls]

coverage: 86.628% (+0.1%) from 86.507%
when pulling c65b6d3 on pgradoboev:encoding-options
into 1d9fc9c on jhillyerd:main.

[jhillyerd]

I like adding the Encoder options.

I don't accept PRs without tests, as subsequent PRs to the same code often end up breaking or reverting that feature. Take a look at this test as a possible starting point:

enmime/encode_test.go

Line 327 in 1d9fc9c

func TestEncodePartContentBinary(t *testing.T) {

[jhillyerd]

Looks pretty good to me, not sure if you're ready for review since it's still marked as draft?

[pgradoboev]

James, I wanted to add more tests after the holidays. But if you think that's enough, then I've changed the request status.

[jhillyerd]

Made a tweak to the test to check a bit of the encoded content.