Allow custom source directories

[kentr0w]

What's done:

Added new configuration param
closes #773

[codecov]

Codecov Report

Merging #778 (6c725ca) into master (517e3d6) will decrease coverage by 0.09%.
The diff coverage is 38.46%.

@@             Coverage Diff              @@
##             master     #778      +/-   ##
============================================
- Coverage     81.28%   81.19%   -0.10%     
  Complexity     2148     2148              
============================================
  Files            97       97              
  Lines          5542     5551       +9     
  Branches       1706     1709       +3     
============================================
+ Hits           4505     4507       +2     
- Misses          271      278       +7     
  Partials        766      766              

Flag	Coverage Δ	Complexity Δ
unittests	81.19% <38.46%> (-0.10%)	0.00 <0.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
...fn/diktat/common/config/rules/RulesConfigReader.kt	34.61% <11.11%> (-4.03%)	4.00 <0.00> (ø)
...qfn/diktat/ruleset/rules/chapter1/PackageNaming.kt	92.04% <100.00%> (+0.09%)	36.00 <0.00> (ø)

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