Expose the reporter API

[nprail]

At the moment, to use npm-audit-html, you have to use it through the CLI. This is inconvenient for some people who need to use it programmatically. For example, with build systems like Gulp.

Additionally, the reporter should be more abstracted. It should not write files it's self but should only return an HTML string. The CLI should do the file writing.

Technically, you could use lib/reporter.js directly but it is currently undocumented and not set as the main file in the package.json.