Fix line numbers and make `rspec -l` work

The problem is `rspec example/errors.feature` reports meaningless line numbers.

```
Failed examples:

rspec ./examples/errors.feature:68 # raises errors Step raises error raise error
rspec ./examples/errors.feature:68 # raises errors Incorrect expectation there is a monster -> it should die
```

Moreover, we cannot use `-l` option to filter examples (scenarios) to be executed.

I fixed this problem by faking line numbers with `instance_eval`.
Please check added specs.