Audit package for vulnerable dependencies

[accraze]

This PR contains a new feature: Auditing a package for vulnerabilities in it's dependencies.

• Using nsp we can audit a package's dependencies for known vulnerabilities and threats listed by the Node Security Project.

The plugin will audit the cwd's package.json file and if nsp finds any vulnerabilities, it will return a SRError otherwise it will return null.

Tests/test data are included

resolves semantic-release/semantic-release#67

[accraze]

@boennemann let me know how this works for you...

[boennemann]

Hey @accraze,

this looks pretty good. I'm just wondering where the packagePath argument comes from, and whether we should just add a default for cwd + 'package.json'.

Best,
Stephan

[accraze]

@boennemann.... I added the default for cwd + 'package.json'... now the packagePath arg is only used for testing.