Add NVM prompt

[alessandro-fazzi]

As it says. Should fix #97

Sincerly I'm not 100% sure about colors I've used for all the themes: I chose the green and applied it down to all the supported color schemes.

Since this is a PR on a project I do not maintain, I decided to have this prompt disabled by default.

If NVM version is deactivated (nvm deactivate) the nvm prompt won't show. I'm open to other/smarter methods to understand if we're not on an nvm's node version.

Documentation should be ok also.

Note: this is a specific PR done for NVM, hence it's not meant to supersede the (stalled?) #128

[alessandro-fazzi]

Sry for the push-force, but I've rebased after a fixup in order to retain clean history: I left a debug echo in the code. My bad.

[alessandro-fazzi]

I've opted to not show the prompt also if system version is used

[bobthecow]

👍

That's the approach I usually take to colors :)

[bobthecow]

Thank you!