Add --update flag with warning messages.

[arcuru]

This change adds a new flag, --update, that will invoke nix-index to update the database.

Along with this change I've added better warning and error messages when dealing with a missing database. The user will now be warned, and told the appropriate remedy, if the database is missing or older than 30 days.

Unfortunately I did need to add another dependency to handle xdg settings correctly. I don't think it added too much size to the binary, but we should probably pay attention to that since this is now a 1MB binary instead of a <100 line bash script.

This was prompted by the UX issues seen in #21, and #23, which were created in #18

[Artturin]

Cool, could you split the index related functions to a separate file?

downloading the prebuilt index instead of building it would be a plus but not necessary in this PR https://github.com/Mic92/nix-index-database

i wouldn't worry about the size as long as its under 100MB

[arcuru]

No problem, I've factored out those helper functions into a separate module but I didn't implement the direct downloads.

Also, please feel free to re-word any of these changes. I'm not that familiar with this area, so I ended up referring to the file as the "nix-index database" but you may prefer to refer to it as just the "index" or something else.

[Artturin]

Please rebase and check #26

[arcuru]

Done.

[Artturin]

Awesome, Thank you!