use captureStackTrace

Cool lib. I noticed a couple issues. However, I noticed you were using `new Error` as prototype of your `GraphError`, this is technically the incorrect way to inherit in JS.

Perhaps you were using it to capture the stacktrace, but the stack trace was incorrect. It was capturing at the `new Error` creation. It should be caught at the point where the `GraphError` was created. Also, the error that is shown in a console doesn't mention 'GraphError'.

Here is a fix for that.

If it throws an error now, the stack trace will be properly generated from the `tap` method.

```
# before
Error
    at ../l33teral.js:40:26

# after
GraphError
    at L33teral.tap (../l33teral.js:93:15)
```