Create new articles in articles instead incoming. Harmonize script naming

[thibaultduponchelle]

Main change

The script new_article does a good job but when it creates in incoming directory it does not make the article immediately ready for generation. I propose to create articles in articles with a date from available ones. It will enable to immediately preview articles by just typing advcal -c advent.ini (just need to force date with --today).

Other changes

• Renaming scripts to match some sort or naming convention + add suffix
• Add shebang + change permissions

[thibaultduponchelle]

@briandfoy

[thibaultduponchelle]

Yes..

There should be a better solution to this.

[briandfoy]

No need to tag me on things. If there's something I've contributed that you need to change, just go for it.