add abiSourceFiles support, add outputJavaParentContractClassName…

[zebraofjustice]

support

What does this PR do?

Closes issue #63

Where should the reviewer start?

README.md file changes

Why is it needed?

Extends capability of plugin usage. ".sol" files are not always provided for wrappers generation. Being able to configure custom parent contract class is also useful.

[codecov]

Codecov Report

Merging #64 (03b20f0) into master (4b4f8ed) will increase coverage by 0.87%.
The diff coverage is 89.74%.

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     85.71%   86.59%   +0.87%     
- Complexity       64       75      +11     
============================================
  Files             6        6              
  Lines           245      276      +31     
  Branches         23       26       +3     
============================================
+ Hits            210      239      +29     
- Misses           25       27       +2     
  Partials         10       10              

Impacted Files	Coverage Δ	Complexity Δ
.../org/web3j/mavenplugin/JavaClassGeneratorMojo.java	85.44% <89.74%> (+1.97%)	41.00 <9.00> (+11.00)

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