This commit moves the --output switch to a regular argument. This aligns
to other command, i.e. spring init my-project.zip would save the project
to "my-project.zip" in the current directory.

This commit also auto-detects the --extract option if the location ends
with a slash, i.e. spring init demo/ would extract the content of the
project in a demo directory that is local to the current directory.

Fixes gh-1802