Timestamp formatting revamp

[Ezrashaw]

I am of the (personal) opinion that the timestamp format used in env_logger is too noisy. So for my personal purposes I rewrote some of the timestamp formatting code to allow for different formats. This is a attempt to share some of that code back to upstream.

Briefly,

• I changed the crate used for timestamp formatting from humantime to chrono, this is to allow for more variance in formats (humantime only supports RFC3339, whereas chrono supports that and more (including custom format strings).
• the default timestamp format remains exactly the same, however it becomes possible to use "human-readable" formats as well.
• This is a reasonably quick-and-dirty implementationm but this is also about changing how the crate does timestamp formatting and not just adding formats.

I'm reasonably new to contributing to open-source Rust projects so I would appreciate a bit of guidance as I know my code is definitely not perfect :P.

EDIT: Obviously, I can do superficial things like adding examples/ unit tests. Just would like some guidance overall first though.

Sorry, I didn't read the contributing guidelines until after I made my commits. Don't know what you want me to do about that.