Allow lib to build for all targets/platforms

[Bayernatoor]

This addresses: #12

PR Adds a hostname feature which activates gethostname dependency and gates it's usage. It adds a #[derive(Default)] to the BunyanFormattingLayer struct so that the default value (<no_hostname>) of field HOSTNAME can be used if the hostname feature is NOT enabled.

This does not change the API and allows the lib to build for any target. It does not allow user to set a hostname (would likely require a change in the public API?)

Let me know if this can be improved/changed in any way.

[Bakrog]

@LukeMathWalker, could you please take a look on this PR?

[Bayernatoor]

Just a heads up that I've been using my own fork with this change for the past 5 months with no issues.