Add a --dry-run option to the tool

[joandrsn]

For new users who are unfamiliar with the code, they might want to explore what the tool can do.

I would suggest adding a --dry-run option for the script.
Example usage:

$ sts classic --dry-run
Username 'abba' would purchase 1x 14 Øl(Grøn/Classic/Rød/Rå) (excl. pant) for 6.67 kr

Another example:

$ sts -u johannes 1837 --dry-run
Username 'johannes' would purchase 1x 1837 Monster / cola energy (excl. pant) for 12.10 kr

More reading on general usage of --dry-run here