[0.x] Add parameter to prefix the route URLs

[xiCO2k]

This PR adds the capability to add a pathPrefix to all the urls.

The main usage will be to people that use proxies like traefik or k8s ingress controller, this way on a domain https://example.com can the be web server and https://example.com/ws will be the pWS server both working on the same port.

Hope this is helpful.
Thanks,
Francisco

[codecov]

Codecov Report

Merging #57 (3fe620f) into master (e5a085d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   86.64%   86.66%   +0.01%     
==========================================
  Files          35       35              
  Lines        1565     1567       +2     
  Branches      262      262              
==========================================
+ Hits         1356     1358       +2     
  Misses        208      208              
  Partials        1        1              

Impacted Files	Coverage Δ
src/server.ts	88.49% <100.00%> (+0.20%)	⬆️

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[willpower232]

@xiCO2k thank you for your efforts, can I ask how you implemented this client side? pusher-js is pretty adamant that nothing should be before /app 😅