Knob to set blank lines near imports, classes, functions and methods

`BLANK_LINE_BEFORE_NESTED_CLASS_OR_DEF` allows adding a blank line before nested classes or methods.

Ideally, a knob would allow setting a specified amount of blank lines near:

* Top-level imports
* Top-level classes and functions
* Classes
* Methods
* Top-level comments

Alternatively, offer a knob can allow "maximum blank lines", thus allowing blank lines after imports (for example). Right now they get formatted away.