Use µs not us

[RobertGauld]

At the risk of starting a debate µ (not u) is the correct SI prefix for micro.

I've deliberatly split the PR into 2 commits incase you want to support parsing but not generating the correct micro prefix.

Additionally based on feedback in #42 a third commit adds a feture flag (new PR as I messed up my branches).

[djc]

Sorry for the delay in response, I'm just starting to help out with maintenance of this crate.

Happy to merge something like this, but this will need a rebase. Please build it as two commits:

• One commit to add parsing for µs
• One commit that adds formatting as µs, disabled by default but enabled with a feature called mu

[djc]

Thanks!