[feat:build] control BundleRenderer shouldPrefetch option

[ghys]

VuePress leaves the BundleRenderer's shouldPrefetch option unset - it used to be () => false but was disabled then removed by @yyx990803 here:

9719bc3#diff-26b681bd4dac10f25d5293c7595c435bL56

The VuePress documentation site I'm building has 400+ pages, prefetching everything is a bit overkill and a bandwidth/server hog. I'd like to suggest a "prefetch" option in options.siteConfig to let the user control the prefetching in config.js. Could be either a boolean or the actual function expected in createBundleRenderer. If unset, the option will not be set and the current behavior will remain.

[ulivz]

PR welcome.