add page_config_hook

[minrk]

• move page_config to get_page_config() method, so it can be overridden (used in NotFoundHandler)
• support settings["page_config_hook"] for customizing page config on a per-request basis. Hook shoud look like:

def page_config_hook(handler: RequestHandler, page_config: Dict) -> Dict:
    ...

closes #219

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[codecov-commenter]

Codecov Report

Merging #220 (17d0810) into master (1a2fdac) will increase coverage by 0.04%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   76.96%   77.00%   +0.04%     
==========================================
  Files          28       28              
  Lines        2071     2079       +8     
==========================================
+ Hits         1594     1601       +7     
- Misses        477      478       +1     

Impacted Files	Coverage Δ
jupyterlab_server/handlers.py	87.80% <91.30%> (-0.02%)	⬇️

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[blink1073]

Thanks!

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