Improve verbosity of updating

[Dentrax]

I think it's worth mentioning to add more verbosity to --update flag since we could not see anything in this first place. How about:

• How many data will be updated?
• Which ones are changed?
• Changed by whom?
• From which commit to where?
• etc.

$ tldr --update
# Nothing writes here

[marchersimon]

The first two points are quite doable. The second two would require to interact with the GitHub API, which is a bit more difficult. If you're interested, I've written my own C++ client, where I have implemented the first two points. That's how I did it:

• Back up all old files
• Create a hash for every file
• Download the new files and also create a hash
• Compare the hashes to see if the file changed

But as I already wrote in #62, this is a client specific issue, so it should be discussed there.