added link command

[Kikobeats]

Useful for link a package as global in your system or link locally a global package.

Based in the documentation of the command:

npm link (in package folder)
npm link [@<scope>/]<pkgname>
npm ln (with any of the previous argument usage)

For cover all cases can be necessary a scope param for specify the process.cwd path, but I'm not sure if you are using options.path at this moment for do something like this.

Anyway we can add something like if is present change the scope with process.chdir, exec the command and revert the process.cwd scope as behavior inside doNpmCommand function.

What do you think? :-)