Move code to packages

[grubastik]

Right now we have everything in main.go. It would be nice to split code by their modules. I see the following packages to be created:

• metrics with code related to Prometheus metrics
• producer with code which covers kafka producer functionality
• parser with code related to parsing url
• fetcher with code related to feeds fetcher
• app with code which is common to the whole app and orchestrates all the pieces

Feel free to add other packages if needed.
It would be nice also to split tests per packages.