_frontend: Simplify color handling

Currently we store color configuration in our App object, and have to remember to pass it around to click.echo() when printing things on the screen. This is error-prone as we can forget to do so. This leads to bugs like #1200 (closed), where bst init was not respecting --no-colors flag.

Instead of doing that, this patch controls colors in output by configuring the click.Context object.

Fixes #1200 (closed).