Improve command organization

[jfontan]

Right now there are two types of producers (from mentions or from file) and there will be a new one soon (repository updates). Check what could be the best way to organize this. Examples:

• Maintain the current producer and add a new one for updates
• Split producer in mentions, from file commands and updates
• Add a new parameter to producer to start the updater

[ajnavarro]

So basically, we will have commands like:

• borges producer mentions to get mentions from rovers and convert them into jobs
• borges producer file get repositories URLs from a file and convert them to jobs
• borges producer errors get jobs that already fails and re-queue them again.

[mcarmonaa]

I'm working on this before address the changes on #250 that require that functionality works as one of this commands.