dmoj-cli does not use shlex

[quantum5]

When a user types in a command into dmoj-cli, the command is not properly parsed shell-style to handle quotes and escapes. Instead, it is simply split using str.split on ' ':

line = command.split(' ')

This makes it very hard to deal with stuff like file names with spaces, for example. As a user, you'd expect a shell-like prompt to be able to handle shell-like quotes. We should just use shlex.split which does all the work for you.