logger.js ignores layout.pattern

### Expected behaviour

In [`karma/lib/logger.js`](https://github.com/karma-runner/karma/blob/master/lib/logger.js#L23), `setup` should use the `layout.pattern` string if it exists.
### Actual behaviour

When `layout.type === 'pattern'`, `setup` only uses one of the built-in constants:

``` js
var pattern = colors ? constant.COLOR_PATTERN : constant.NO_COLOR_PATTERN
```
### Environment Details
- Karma version (output of `karma --version`): 1.3.0
- Relevant part of your `karma.config.js` file

``` js
loggers: [
{
type: 'console',
layout: {
type: 'pattern',
pattern: '%[%d{ISO8601}:%p [%c]: %]%m'
}
}
]
```
### Steps to reproduce the behaviour
1. Supply a custom logger in `karma.config.js` as specified above.
### Comment

If this is indeed broken, I can submit a PR.

Further, I realize that fixing this to use the provided `pattern` will then ignore the `colors` boolean. This might be addressed by having `layout.colorPattern` and `layout.noColorPattern` which get assigned appropriately to `layout.pattern`. I can submit a PR for this as well.

Thanks.

Start in karma/lib/logger.js at setup and compare the pattern layout handling with the supplied karma.config.js example. Reproduce the issue with a custom console logger and verify that the configured layout.pattern is used, including the interaction with the colors setting.

Written by the indexing model from the issue text.