Support for custom validators

[vnenkpet]

Adds support for custom validators. Sync/async functions or regex.

Example (added to README.md):

// you have to get your own `isEmail` function, this is a placeholder

@prop({ validate: (value) => isEmail(value)})
email: string;

// or

@prop({ validate: (value) => { return new Promise(res => { res(isEmail(value)) }) })
email: string;

// or

@prop({ validate: { 
    validator: val => isEmail(val), 
    message: `{VALUE} is not a valid email`
}})
email: string;

// or

@prop({ validate: /\S+@\S+\.\S+/ })
email: string;

// you can also use multiple validators in an array.

@prop({ validate: 
    [
        { 
            validator: val => isEmail(val), 
            message: `{VALUE} is not a valid email`
        }, 
        {
            validator: val => isBlacklisted(val), 
            message: `{VALUE} is blacklisted`
        }
    ]
})
email: string;

[coveralls]

Coverage remained the same at 87.009% when pulling 6b524ce on vnenkpet:pr-custom-validators into 24b66fd on szokodiakos:master.

[vnenkpet]

I'm not sure how much is this repo maintained but it would be nice to have this feature as it's a default mongoose feature that is missing for a reason I don't understand.

All I did could be also achieved by simply adding validate?: any to the BasePropOptions interface in src/prop.ts. I just added some type checking and updated README on top of that, also included one use case in tests.

I'm already using this in my projects and it works without a problem so far.