let subclass of ConfigOption be able to override parent options

[javeme]

also implement ConfigOption.toString()

Change-Id: I1f1fd5eeaa80252fc093e7cba5af525a32a23c99

[codecov]

Codecov Report

Merging #24 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #24      +/-   ##
============================================
+ Coverage     72.85%   72.94%   +0.08%     
- Complexity      437      439       +2     
============================================
  Files            44       44              
  Lines          1507     1508       +1     
  Branches        228      228              
============================================
+ Hits           1098     1100       +2     
  Misses          302      302              
+ Partials        107      106       -1

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/baidu/hugegraph/config/ConfigOption.java	74.46% <100%> (+0.55%)	13 <1> (+1)	⬆️
.../java/com/baidu/hugegraph/config/OptionHolder.java	76.92% <100%> (ø)	5 <0> (ø)	⬇️
.../main/java/com/baidu/hugegraph/perf/Stopwatch.java	87.75% <0%> (+2.04%)	21% <0%> (+1%)	⬆️

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