function snakeCase(subject) {
    var subjectString = coerceToString(subject);

    if (subjectString === '') {
      return '';
    }

    return words(subjectString)
      .map(lowerCase)
      .join('_');
  }
Converts the `subject` to <a href="https://en.wikipedia.org/wiki/Snake_case">snake case</a>.

@function snakeCase
@static
@since 1.0.0
@memberOf Case
@param  {string} [subject=''] The string to convert to snake case.
@return {string}              Returns the snake case string.
@example
v.snakeCase('learning to fly');
// => 'learning_to_fly'

v.snakeCase('LearningToFly');
// => 'learning_to_fly'

v.snakeCase('-Learning-To-Fly-');
// => 'learning_to_fly'