Improve createapp CLI by adding options

[fer22f]

Issue

• No way to stop createapp from automagically choosing between npm and yarn, and the installing process is quite slow (and may be skipped).
• No way to stop createapp from creating a git repository automagically (other than uninstalling git). Altough git is desirable, you can use others (Mercurial, subversion, who knows).
• No way to stop createapp from overwriting existing directories.
• CLI also has a bug where if you specify options before unnamed options (such as ), it shows unneeded usage information, while the command is running.

Solution and steps

• Add options for these, and for overwriting existing directories, abort the operation if the directory exists.
• Fix CLI bug.

Checklist

• Add/update/check tests.

(currently not automatically tested, but has been manually tested, we can discuss this and work on more CLI improvements over time).

[LoicPoullain]

Thank you for submitting this PR and for providing this clear description. Some things to improve and we're good 👍