Fix to work on maven multi-module project

[duck8823]

I tried using detekt-maven-plugin is introduced by README in this repository.
But, unfortunately, it didn't work in my multi-module project.

I made a sample here.
The detekt task success first module, but second module occurred error Can only specify option -i once..

I found that jcommander instance is defined as global variable.
Because the jcommander instance is used multiple times in multi-module project, argument parse error occur!

In this PR, I suggest that jcommander instance define as local variable.

[arturbosch]

Hey good catch! This is a leftover from a refactoring, thanks for contributin!