this commit adds more prompt components for arangosh
the following components are now available for dynamic prompts, settable via the `--console.prompt` option in arangosh:
- '%t': current time as timestamp
- '%p': duration of last command in seconds
- '%d': name of current database
- '%e': current endpoint
- '%E': current endpoint without protocol
- '%u': current user
The time a command takes can be displayed easily by starting arangosh with `--console.prompt "%p> "`.