feat: allow default lean options plugin config

[DesignByOnyx]

Summary

This allows setting { getters: true } at a plugin level. This can be overridden at call time to disable getters if need be. Closes #33.

Examples

const testSchema = new mongoose.Schema({
  field: {
    type: String,
    get(val) { return `${val}-suffix`; },
  }
});
testSchema.plugin(mongooseLeanGetters, { defaultLeanOptions: { getters: true } });

const TestModel = mongoose.model('gh-33', testSchema);
const entry = await TestModel.create({ field: 'value' });
const doc1 = await TestModel.findById(entry._id).lean();
assert.equal(doc1.field, 'value-suffix');

const doc2 = await TestModel.findById(entry._id).lean({ getters: false });
assert.equal(doc2.field, 'value');