Refactor upload script

Hello,

I have refactored the upload script. Previously, we could only change the language and the user, and these were supplied as postional arguments (which is awkward when we want to change the user but not the language). I did that using `getopts`, which is a Bourne shell builtin and therefore portable. I added some flags and updated the help message. It isn't, of course, backwards-compatible but I still think it makes the upload script much more flexible.  
Because a short help message is worth a thousand words, here's the usage message of the new script:

```
Usage: ./paste.sh [-s] [-l language] [-u user] [-c] [ -C copy_command ] [ -p pastebin_url ] file
 -s: toggles silent mode (do not print the final URL)
 -c: copies the final URL to clipboard
```

Thanks in advance for your consideration,  
Matthias Rabault