[WIP] Expand use of requireFirst to runners

[kkoskelin]

... if runner is not found

also, package updates.

Addresses #529, in part, dealing with "bad runner" scenario. broader CLI validation still needed.

[kkoskelin]

Output is now:

$ node pa11y.js --runner potato https://www.nature.com

Welcome to Pa11y

 > Running Pa11y on URL https://www.nature.com
(node:13129) UnhandledPromiseRejectionWarning: Error: Runner module not found: potato
    at loadRunnerFile (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:466:9)
    at loadRunnerScript (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:497:23)
    at injectRunners (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:350:39)
    at runPa11yTest (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:144:8)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async pa11y (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:39:18)
    at async runProgram (/Users/kkoskelin/projects/pa11y/bin/pa11y.js:133:19)
(node:13129) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:13129) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:13129) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)

Error: Runner module not found: potato
    at loadRunnerFile (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:466:9)
    at loadRunnerScript (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:497:23)
    at injectRunners (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:350:39)
    at runPa11yTest (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:144:8)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async pa11y (/Users/kkoskelin/projects/pa11y/lib/pa11y.js:39:18)
    at async runProgram (/Users/kkoskelin/projects/pa11y/bin/pa11y.js:133:19)

Unless you would want me to export the loadRunnerFile from lib/pa11y.js to bin/pa11y.js so that we can attempt to import it there for purposes of verification, only to drop it on the floor and allow it to be required a second time by lib/pa11y.js?

[joeyciechanowicz]

This all seems very sensible, nice one!