Change escaping method from Shellwords.escape to simply quoted strings, ...

Using `Shellwords.escape` introduces a bug on Windows, where methods like `transcode` and `screenshot` will fail if there are spaces in either the input or output filenames. `Shellwords.escape` uses `\` as its escape character, so space become `\ ` -- but on Windows, `\` is the folder delimiter in path names and the caret `^` is the escape character instead. Switching to a simple quoted string lets those method calls work on any OS.