Add client-side playback as option to pywb

[tw4l]

Sub-tasks:

• Bundle wabac.js serviceworker with pywb (perhaps similar to how wombat.js is included) and configure pywb to serve the serviceworker with an appropriate Service-Worker-Allowed header or from a root path (e.g. /sw.js)
• Add and document a configuration option to use wabac.js for HTML rewriting instead of pywb's server-side rewriting. This option should initially be off but the aim should be to enable it by default once there's been some more testing and any major regressions have been resolved
• Documentation for CORS configuration