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.