Added autofix function for operator-whitespace rule

[ihordiachenko]

Related Issue

#94

Changes

This MR includes autofix for all operator-whitespace cases and a refactoring of the rule implementation itself.

Major changes:

• reduced usage of regexps for performance and readability
• reduced nesting of code and merged some redundant code branches
• comments are treated more carefully: operators inside comments are escaped before processing
• makes use of ES6 features

Declaration

• I have followed the Contribution Guidelines.
• All tests are passing.
• ESLint doesn't produce any warnings.
• ESLint doesn't produce any errors.
• I'm awesome.

NOTE: It is fine if ESLint does produce warnings for your PR if there is a good reason for it.

[ihordiachenko]

Hi, @duaraghav8 Could you review it please?