Scan and use plugin generators

[KristianKarl]

This is a proposal to fix the issue discussed in New path generator to be recognized when defining the generator

I am implementing two new graph traversal algorithm so new path generators. I am wondering where can I set these new path generators to be recognized in the GraphWalker.
For example in the model.json file when I change the generator name to the new path generator I am implementing. -> "generator":"mynewgenerator(edge_coverage(100))"
Or
In the studio when we set the Generator field under execution tab like mynewgenerator(edge_coverage(100)) where in both cases "mynewgenerator" is my new path generator I want to be applied.

This PR would be able to scan for generators in the classpath and use them. Studio or CLI would the be run like:

java -cp graphwalker-cli-4.1.0-SNAPSHOT.jar:myLib.jar org.graphwalker.cli.CLI offline -g SimpleModel.json

Where myLib.jar contains a new [class] generator PluginGenerator, and the model SimpleModel.json has the generator defined as:

"generator": "pluginGenerator(edge_coverage(100))",

[codecov-io]

Codecov Report

Merging #216 into master will decrease coverage by 0.09%.
The diff coverage is 75%.

@@             Coverage Diff             @@
##             master     #216     +/-   ##
===========================================
- Coverage     76.07%   75.97%   -0.1%     
- Complexity     1187     1195      +8     
===========================================
  Files           127      129      +2     
  Lines          4368     4400     +32     
  Branches        656      660      +4     
===========================================
+ Hits           3323     3343     +20     
- Misses          814      822      +8     
- Partials        231      235      +4

[KristianKarl]

@nilols Good to go?