Display line numbers with errors (in `Vader!`) / setup 'errorformat'

While Vader populates the quickfix list itself when run inside of Vim, the output from `:Vader!` does not contain information about the line number.

In this context it would be also useful to ship an `errorformat` setting that allows to parse this, e.g. in the documentation.

What I've came up with is the following:

```
let &l:errorformat='%+P Starting Vader: %f,%+E%.%#(X) %m,%Z,%-G%s'
```

It gets the filename from the "Starting Vader:" line.

Some explanation:
- %P single-line file message: push file %f onto the stack
- %f file name (finds a string)
- %E start of a multi-line error message
- %m error message (finds a string)
- %Z end multiline message
- %-G%s ignore everything else

I've used %E (and %Z to end it) to allow for `%+E` to include the whole line.

With `%+P` and `%+E` the `+` means to use the whole line in the output.

(see `:h errorformat`)