This adds a new directory and file option named lookupMap which allows the supported content-encodings and file extension for lookupCompressed to be customized.

The primary reason for this, is to enable non-gzip lookups once hapi@15 lands with custom encoding support. Eg. it would be possible to lookup brotli pre-compressed files with this config:

{
    lookupCompressed: true,
    lookupMap: { gzip: '.gz', br: '.br' }
}

Note that the compressed file will not be served, unless the server already understands how to compress the br encoding, via https://github.com/kanongil/brok or similar.