Get unit names (singular and plural)

[goldenphoenix713]

Hello, I've recently started looking for an open source project to contribute to, and I found this library when looking for unit support for something I'm doing at my work.

One thing that I've been looking for is getting the unit names as well as their symbol as defined when using new_unit. This is a feature that I figured would be useful for some of the work I'm doing. I also wanted to have the plural form for each unit name as well.

I cloned the repo over to my account, and I've been making some of these improvements. Does this sound like something you'd be interested in for me to merge back. If not, I can just keep the changes over on my account. I can also contribute elsewhere as needed.