feat: add .nmvrc check and arg-less nvs add

[RichiCoder1]

It looks like you already did a ton of good work in NodeVersion to ensure nvm compatibility, so it was just a matter of checking for the presence of an .nvmrc and attempting to use that version.

Let me know if you think this is good and I'll add test/docs.

Closes #113;

[jasongin]

Looks good. Tests and docs would be nice too!

I have one concern: nvs does not support 100% of the same version syntax as nvm. The one major difference I'm aware of is how with nvm you can specify lts/* (for latest lts) or lts/carbon (for latest node 8.x).

Edit: Wait, now that I checked the code I see I did code that compatibility already. Nevermind!

[RichiCoder1]

Edit: Wait, now that I checked the code I see I did code that compatibility already. Nevermind!

Yup! It looks like you already cover pretty much every readonable .nvmrc case :).

I'll add some docs and tests in the morn!

[RichiCoder1]

Added a light test and some docs & comment updates

[jasongin]

Looks good, with just those small doc edits.

Thanks!