Make gethostname an optional feature

[daniel-levin]

Hi there @mdecimus!

I am a happy user of several of your email crates, including mail-builder. I want to be able to use mail-builder without depending on gethostname. I propose making the dependency on gethostname optional, but enabled by default, so that existing users of this crate are not disrupted in any way.

This change has three benefits:

1. It reduces the size of the dependency graph.
2. MessageBuilder never blocks, so it's usable inside an async context.
3. My outputs become deterministic and aren't tied to the machine they're generated on. This is important for my tests in my network-jailed CI system.

While the usage of #[cfg(...)] inline is a bit awkward, I found it was the simplest way to achieve my goal. The alternatives were:

1. Using a macro, which means you have to navigate elsewhere to see what the code is really doing.
2. Needlessly copying the &str returned by gethostname.

[mdecimus]

Thanks!