Make the creation of a symlink an overridable default behaviour

The creation of a symlink kind of caused quite a bit of headaches because we build systems where NVM is installed (and managed) by superusers and used by a whole bunch of normal (or system) users. 

This PR would allow us, on such system, to at least disable a feature that although doesn't technically hurt us or have much of an effect, generates errors on screen at login time or run time, like so:

```
$ nvm use v0.123.456
rm: cannot remove `/opt/nvm/current': Permission denied
Now using node v0.123.456
```

(I know, I am using the future of Node :D)