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.

[ptdecker]

If this is added, can it be done as a feature switch? I have a very significant corporate customer (a large global financial institution) who has dinged us (a small start up) for delivering logs that contain µ. I'm using this in a CLI application because it is so awesome but if it starts outputting the unicode symbol again they will freak out. Thank you for considering the welfare of your fellow coders! ;-)

[RobertGauld]

Thanks @ptdecker, that suggestion's in #45