Enable linting
This adds a "lint" script to package.json which runs eslint on the "bin", "src" and "test" directories.
It also adds linting as a "pretest" script, so tests will fail if there are linting errors.
It's currently using "extends": "eslint:recommended"
rather than "extends": "standard"
as a recent change somewhere in either eslint or standard has created an error; once that's fixed we can use "extends": "standard"
again.