303 nvm in fish

Hi,

Related to #303 , I use fish myself and love that I can run `npm install -g <...>` with nvm --> fish nvm was needed.

This is simply a wrapper around nvm for fish. I have commented my commits as much as possible to help understand my thought process. I hope others with fish can test this (tests in vagrant worked for me).  
A wrapper allows nvm to change below without (hopefully) affecting functionality in fish.

I'm open to critical feedback or improvement suggestions.

Cheers
That same as before. The change I made shifts all args left:
```shell
# nvm [ use --optional version ]
# $@ = [use --optional version]  <-- this was checked before
shift
# $@ = [ --optional version] <-- now this is checked
```

If the call is just `nvm` the args will be treated in the enclosing [default case line 1070](https://github.com/LoveIsGrief/nvm/blob/303_nvm_in_fish/nvm.sh#L1070).

If the call is just `nvm use` it will be treated [right after the shift](https://github.com/creationix/nvm/pull/579/files?diff=unified#diff-e1e8e036966ad5aaf7709712cd5d8ee3R799) --> .nvmrc

The other check was moved into the following [while>case>default](https://github.com/creationix/nvm/pull/579/files?diff=unified#diff-e1e8e036966ad5aaf7709712cd5d8ee3R815).