Improvements on temp dir handling in case of cancellation or failure

[aersam]

The temp dir could be improved for the case of cancellation of the generation process. If I cancel ng-openapi-gen there are some issues:

• The temp dir still contains files. If I did not "gitignore" the directory, I might accidently commit this. Therefore, please document the usage of the temp dir and allow to configure it's location
• As the temp dir contains files, the next generation will fail unless I delete the directory myself. Therefore, one could clean the generated directory before generating code

As always, I'd do a PR if you want.

[luisfpg]

@aersamkull Please, try this commit. I've done several tests and for me the temp dir is always deleted now.