Add line_wrap to encoding config

[accidentaldevelopment]

This PR adds a line_wrap field to the EncodeConfig struct that determines the line length for the encoding.

Unfortunately this is a breaking change. To avoid it being a breaking change in the future, I made the fields private and added chain-able setter methods. Adding fields going forward would not be a breaking change, just a new feature.

After writing this, I came up with another idea that would avoid a breaking change. I could create a struct Encoder that essentially has the same state as EncodeConfig but private. Encoder::encode(&self, &pem) could handle encoding a PEM with the specified config without requiring any changes to the existing encode function or EncodeConfig struct.

I can put up a separate MR with that change if that is better, or if you'd like to compare.

Resolves #48

[jcreekmore]

What you did is just fine. That is the approach I was going to take.

It looks like a sub-sub-sub dependency for testing is breaking that MSRV test. I am trying to figure out if I can make that failure go away because I don't think I even need the crate pulled in that moved forward to 1.63.

[jcreekmore]

OK, I fixed up master. Can you rebase this on top of master and also add a CHANGELOG entry? Maybe something like "make EncodeConfig struct extendable and add a line_wrap config option"?

[accidentaldevelopment]

Done and done!

[accidentaldevelopment]

Doc tests, always fun :). Fix incoming.

[jcreekmore]

I went ahead and published 3.0.0 with this change.

[accidentaldevelopment]

That's amazing, thank you the quick turnaround!