Make LabConfig configurable and add config API docs

[afshin]

Continuation of #169

[codecov-io]

Codecov Report

Merging #172 (846977c) into master (8d127b2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   74.70%   74.70%           
=======================================
  Files          21       21           
  Lines        1668     1668           
=======================================
  Hits         1246     1246           
  Misses        422      422           

Impacted Files	Coverage Δ
jupyterlab_server/config.py	62.50% <100.00%> (ø)

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[blink1073]

Thanks!