Create new checker for every jscs target run

[markelog]

Say, for example, you have this in Gruntfile

jscs: {
    main: "main.js",
    secondary: "secondary.js"
}

You don't have errors in main.js, but you do have one in secondary.js.
Then, you run grunt jscs command without specifying the target, as a result, both targets will be run.
In stdout you get three absolutely identical errors found in secondary.js, instead of one.

This would happen because

jscs.registerDefaultRules();
jscs.configure( require( cfgPath ) );

lines will be executed twice, whereas Checker constructor (which is initialized from out of context of jscs task) saves the rules in the returned object, so they will be augment every time.

This commit resolves the problem.

[gustavohenke]

Thanks for the fix!

[gustavohenke]

0.2.0 published with this fix.