move ci to github actions

[mainrs]

This adds a first draft of a GitHub Actions workflow that does some basic checks on PRs:

• Run rustfmt
• Run clippy
• Run check on all available Rust versions (beta, nightly, stable) + MSRV of 1.31.0
• Run ci crate and example

Relies on #165 to successfully run CI.

Edit: I am not sure how to actually test run this, I will probably just create a dummy PR on my fork and report back once they run successfully.

Edit 2: I created a (really ugly) PR on my fork at https://github.com/SirWindfield/env_logger/pull/3/. All checks pass except the lint, which is expected.

[mainrs]

Ok, so it all works. There is some stuff I want to point out though:

1. Clippy runs against nightly and not 1.31.0, meaning that it picks up lints that are currently not supported on 1.31.0 (like #[non_exhaustive]). This should be changed, shouldn't it?
2. Documentation is currently not published to github pages. Not sure if this is actually needed or not. I do think a workflow that listens for tag pushes and automatically publishes to crates.io is more beneficial here.

[KodrAus]

Thanks @sirwindfield! This looks great to me. I think for clippy running against nightly is ok, because we should be able to #[allow(clippy::lint)] without producing warnings on channels that we can't implement the lint for. What do you think?

[mainrs]

Sounds like a good plan. We can fix those later on if the MSRV is bumped up :)

I'd however like to open up an issue for adding a cd workflow as well to make publishing the crate a little bit more automated :)
Done.

[KodrAus]

This looks good to me!