Improve Readability.FunctionNames

[sasa1977]

Adds some fixes and improvements to Readability.FunctionNames:

• functions defined with parentheses are now verified (previously this check was only verifying a function if it accepts zero args and is defined without parens, i.e. def some_fun do end)
• functions with guards (when clause) are properly handled
• verifies private macros and custom guards
• generates single report for multiclause functions

[rrrene]

Thx @sasa1977 👍