logging: allow custom ANSI escapes

When I first started with functions, my immediate thought was to improve my own experience by using [chalk](https://www.npmjs.com/package/chalk) to manually color my terminal output – errors would be red, warnings yellow, and normal/successful logs default terminal colors.

However, it seems that it's relatively impossible to do this in the context of building a Function that uses `context.log()` and uses chalk to build out the "colors" of that output. More precisely, it seems that custom ANSI escapes are stripped.

It would be a nice DX improvement to enable folks to customize their log output by enabling custom ANSI escapes.