clarify output of offline command

[mykmelez]

The offline command's output could be clearer. Here's the current output:

[16:15:17] Caching static resource "dist/CNAME" (13 B)
[16:15:17] Caching static resource "dist/css/index.css" (378 B)
[16:15:17] Caching static resource "dist/favicon/android-chrome-144x144.png" (2.65 kB)
[16:15:17] Caching static resource "dist/favicon/android-chrome-192x192.png" (3.65 kB)
[16:15:17] Caching static resource "dist/favicon/android-chrome-36x36.png" (692 B)
[16:15:17] Caching static resource "dist/favicon/android-chrome-48x48.png" (839 B)
[16:15:17] Caching static resource "dist/favicon/android-chrome-72x72.png" (1.23 kB)
[16:15:17] Caching static resource "dist/favicon/android-chrome-96x96.png" (1.67 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-114x114.png" (2.06 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-120x120.png" (2.15 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-144x144.png" (2.73 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-152x152.png" (2.9 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-180x180.png" (3.47 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-57x57.png" (1 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-60x60.png" (1.05 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-72x72.png" (1.25 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-76x76.png" (1.29 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon-precomposed.png" (4.87 kB)
[16:15:17] Caching static resource "dist/favicon/apple-touch-icon.png" (3.47 kB)
[16:15:17] Caching static resource "dist/favicon/browserconfig.xml" (383 B)
[16:15:17] Caching static resource "dist/favicon/favicon-16x16.png" (522 B)
[16:15:17] Caching static resource "dist/favicon/favicon-194x194.png" (4.87 kB)
[16:15:17] Caching static resource "dist/favicon/favicon-32x32.png" (827 B)
[16:15:17] Caching static resource "dist/favicon/favicon-96x96.png" (2.3 kB)
[16:15:17] Caching static resource "dist/favicon/favicon.ico" (15.09 kB)
[16:15:17] Caching static resource "dist/favicon/manifest.json" (727 B)
[16:15:17] Caching static resource "dist/favicon/mstile-144x144.png" (3.74 kB)
[16:15:17] Caching static resource "dist/favicon/mstile-150x150.png" (3.75 kB)
[16:15:17] Caching static resource "dist/favicon/mstile-310x150.png" (4.07 kB)
[16:15:17] Caching static resource "dist/favicon/mstile-310x310.png" (8.44 kB)
[16:15:17] Caching static resource "dist/favicon/mstile-70x70.png" (2.52 kB)
[16:15:17] Caching static resource "dist/img/pause.svg" (429 B)
[16:15:17] Caching static resource "dist/img/reset.svg" (488 B)
[16:15:17] Caching static resource "dist/img/start.svg" (278 B)
[16:15:17] Caching static resource "dist/index.css" (223 B)
[16:15:17] Caching static resource "dist/index.html" (3.04 kB)
[16:15:17] Caching static resource "dist/index.js" (4.29 kB)
[16:15:17] Caching static resource "dist/js/index.js" (8.43 kB)
[16:15:17] Caching static resource "dist/js/offline-manager.js" (176 B)
[16:15:17] Caching static resource "dist/scripts/offline-manager.js" (176 B)
[16:15:17] Total precache size is about 102.14 kB for 40 resources.

This is mostly alright, except that static resource is unnecessary (they're all static resources on GitHub Pages), and the timestamps aren't useful either. I might consider adding checkmarks, as we've done for sequences in #148, #160, and #161.

I might also consider adding some header and footer text, although offline is different from configure, bootstrap, and integrate in that it gets run frequently and automatically (as part of a build sequence), whereas those other commands get run infrequently and always manually. So I'm not sure it makes sense to include a header/footer. At most, I would add a one-line header like:

Offlining [TARGET DIRECTORY] to [path/to/offline-worker.js]…