Strip HTML from CLI output

It's a bad practice to include HTML in CLI output. It clutters results to the point of illegibility on occasion. However, since this same output ends up in our task logs, it can be very useful to use placeholders in dt() that will format strings for easier parsing.

Thankfully, Drush's logging capabilities provide us with a relatively simple way to get the best of both worlds, using a custom log callback.

Patch forthcoming.