Cannot use in worker because window is not defined

In `src/utils.ts`, the [console methods are cached](https://github.com/logdna/logdna-browser/blob/2f42857457fa8be92fdc8583379b1bb3f9f7331c/src/utils.ts#L96-L113). However, in a JS worker, `window` is undefined, and, so, this code fails with "ReferenceError: window is not defined".

It appears these cached methods are only used in two places: [src/plugins/logger.ts](https://github.com/logdna/logdna-browser/blob/2f42857457fa8be92fdc8583379b1bb3f9f7331c/src/plugins/logger.ts#L24) and [src/capture.ts](https://github.com/logdna/logdna-browser/blob/2f42857457fa8be92fdc8583379b1bb3f9f7331c/src/capture.ts#L80).

Start in src/utils.ts at the cached console methods, then inspect their two reported uses in src/plugins/logger.ts and src/capture.ts. Verify the behavior in a JavaScript worker where window is undefined; done means the worker no longer fails while those logging and capture uses remain functional.

Written by the indexing model from the issue text.