Fix and improvement to thrush

The initial commit has some bugs (`partial` has arguments reversed, the initial call of `apply-task` is missing an argument) and is not fully in line with the `do` task and the `->` operator (doesn't allow more arguments besides the threaded one, doesn't use comma-separated argument groups). I propose to fix and improve the task with this commit. I also took the liberty to rewrite the docstring by copy-pasting the description from `defproject`, which seems more expressive, and a relevant sentence from the `do` task.