Add serde impls to LogLevel and LogLevelFilter

[aergonaut]

Fixes #143

Adds serde as an optional dependency and puts the Serialize and Deserialize traits behind cfg_attr. I wasn't sure what version I should specify. 1.0.7 was the latest when I checked, but maybe it's too specific? Would >= 1.0 be better?

Saw the post on the internals forum, wanted to help out!

[alexcrichton]

Thanks! Could you also add some CI configuration to test this on Travis/AppVeyor? Also, I think the dependency here is safe to just be serde = "1.0" as >= is actually a little too eager as it may or may not be compatible with 2.0 (we're not sure if it will be).

[dtolnay]

@aergonaut are you still working on this or would you need to hand it off to someone else?

[aergonaut]

I would still like to work on this! I will try to come back to it by the end of the week.

[dtolnay]

No problem, let us know if you get stuck.

[aergonaut]

@dtolnay @alexcrichton I think this is ready now! I added cargo test --features serde to the Travis and Appveyor configs. I also fixed up the feature flag usage so it actually works.

[dtolnay]

Thanks, I combined this with #181.

[dtolnay]

Thanks, I combined this with #181.