Respect gulp --silent?

Hi, would it be possible to have the default `quiet` option based on the `--silent` flag of gulp's cli?

According to [this comment](https://github.com/gulpjs/gulp/issues/499#issuecomment-93824634) the setting can be found by doing:

```
const argv = require('minimist')(process.argv.slice(2));
const isSilent = !!argv.silent;
```

then the change could be as simple as replacing

```
opt.quiet = bool(opt, 'quiet', false);
```

by

```
opt.quiet = bool(opt, 'quiet', isSilent);
```

If that sounds reasonable I'll add some tests, try it out and submit a pull request when I have time.

Locate the option handling that currently uses `opt.quiet = bool(opt, 'quiet', false)` and review how gulp CLI arguments are available through `process.argv`. Add tests as proposed in the issue, then verify that `--silent` supplies the default quiet value while an explicit quiet option still behaves correctly.

Written by the indexing model from the issue text.