Read the report configuration file from the options

This PR is very similar to #179

Reports are already trying to read the destination file name from the options, but this value is always undefined (except in tests perhaps).

See for instance the cobertura report

```js
function CoberturaReport(opts) {
    Report.call(this);
    opts = opts || {};
    this.projectRoot = process.cwd();
    this.dir = opts.dir || this.projectRoot;
    this.file = opts.file || 'cobertura-coverage.xml';  // <-- look, here
```

This pull requests propagates the value `opts.file` from `.istanbul.yml` to reports, giving the possibility to write to a different file name.

Btw, I've tried with

```
istanbul report --file coverage.xml cobertura
```

but this does not generate any report at all