Add `maskPattern` option to allow overriding rather than selecting

[shabbyrobe]

We are hoping to be able to take advantage of the inherent ability of QR codes to produce multiple different versions of the same QR. We have found that all 8 versions of any QR we have tested scan just fine on modern devices, so the "automatic best mask" does seem to be a bit of a relic of a bygone era anyway.

This PR exposes this facility by introducing an option to allow the overriding of the mask pattern like so: qrcode("abcd", { maskPattern: 2 }).

This PR has a rather large diff --stat as I removed the coverage files so as not to radically bloat this PR with lines changed, but can re-add them if you'd prefer they stay. I have also added the testing tools as devDependencies, which introduces a package-lock.json.