Add option to output day and month literals instead of numbers

[nathcouret]

The current implementation can parse both literals and numbers, but the generated AST only outputs can only outputs day and month expressions with numbers.

Option should defaults to false.

Sample code

Proper syntax to determine

const expression = "0 0 * * MON-FRI";

const numberResult = cronix(expression);
// outputs 1-5
console.log(numberResult.ast.dow.value());

const literalResult = cronix(expression, {outputLiterals: true});
// outputs MON-FRI
console.log(literalResult.ast.dow.value());