Adds mocha-aware prefer-arrow-callback rule

[quarklemotion]

Addresses #141

---

• Copied core eslint prefer-arrow-callback rule code, test code, and markdown doc from: https://github.com/eslint/eslint/blob/master/docs/rules/prefer-arrow-callback.md
• Modified this rule to be mocha-aware by utilizing the existing utility function astUtils.isMochaFunctionCall(node, scope)
• Updated tests to include rule not triggered tests for all mocha functions
• Updated documentation to include instructions to disable the core prefer-arrow-callback rule before enabling the mocha/prefer-arrow-callback rule

The following eslint rules are failing with this code:

/Users/mfields/workspace/eslint-plugin-mocha/lib/rules/prefer-arrow-callback.js
   90:1   error  Function 'getCallbackInfo' has a complexity of 12                                     complexity
   90:1   error  Function 'getCallbackInfo' has too many statements (13). Maximum allowed is 10        max-statements
  251:38  error  Method 'FunctionExpression:exit' has a complexity of 16                               complexity
  251:38  error  Method 'FunctionExpression:exit' has too many statements (14). Maximum allowed is 10  max-statements
  290:28  error  Method 'fix' has a complexity of 9                                                    complexity
  290:28  error  Method 'fix' has too many statements (11). Maximum allowed is 10                      max-statements

It looks like it will take some substantial work to get this code in line with the complexity limit of 4. I think it would be preferable to not completely rewrite this rule so that updates from the core eslint prefer-arrow-callback rule can be applied more easily here in the future. Would the maintainers here be open to a rule-specific relaxing of the complexity and max-statements eslint rules (now included in code changes)?

[lo1tuma]

Nice work, LGTM 👍.
Also the reasoning for relaxing some eslint rules in this specific file makes sense.