Update to edition 2018, deps and docs

[estk]

Closes #105
Closes #106

[estk]

@calvinbrown085 I would love a review on this.

[calvinbrown085]

@estk Now that we are on 2018, can we get rid of some of the extern crate * lines in lib.rs ?

https://github.com/estk/log4rs/blob/master/src/lib.rs#L170

For example:
extern crate arc_swap;
or
extern crate log;

If we aren't using any macros from the library I believe we can get rid of them, but I could be wrong there :)

[calvinbrown085]

@estk I think my only question was #108 (review)
Otherwise lgtm!

[estk]

@calvinbrown085 thanks for the review! Removing the extern crate stuff is a little harder than I thought, but its in the works. Thanks again!

[calvinbrown085]

@estk You're welcome!

[calvinbrown085]

@estk Anything I can help out with here?

[estk]

@calvinbrown085 I'm slammed with the holidays coming up. I'm inclined to merge this as is and open an issue to remove the extern crate stuff. How does that sound? Also, if you feel like working on the extern crate work I would love that!

[estk]

#113 addresses the extern crate stuff, not sure its ready to be shipped though

[calvinbrown085]

@estk Yeah totally understandable! I can help with the extern crate stuff. I think this is ready to 🚢

[calvinbrown085]

Also, apologies if that came off as forceful!