Added prompt for when zola build output-dir mentions an existing directory.

[coastalwhite]

I went about implementing this. It wasn't too hard, especially since ask_bool and Path::exists exist.

There is one concern I ran into after implementing this. It may need to be discussed still. Is it okay to require CI's to first delete the folder in question? This may break CI's and automated scripts, and therefore it may be interesting to add to the 0.15 release.

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Added ask_bool prompt for --output-dir for when the output directory targeted already exists in the file system. Updated the documentation to match this behaviour.

Fixes #1378.

[Keats]

Seems ok but I'll wait to see if there is a need for a 0.14.1 before merging that.

[coastalwhite]

There is now a 10s prompt timeout using tokio's timeout.

[Keats]

Thanks!

[Yaroslav-95]

So I am back here to complain about another change that breaks my use-case :) I want to propose a flag that will force the build without prompting. My rationale is the following: I have a git hook that builds my site on every push to master and, although I could edit the script to delete the folder prior to building, I don't want to that, because I need to preserve the mode bits and ownership of the directory. What do you say @Keats ?

Actually I already have a patch, so I'll create a PR in a little bit ;)

(Edited to add some clarity into my words)