Add option to show total visits

[yurihs]

Hi! This should resolve #128.

Please let me know if there are any problems.

[stklcode]

Thanks very much @yurihs for this work.
I've only had a look into the code so far. Implementation looks good to me.

One additional suggestion though:
"all time" is quite a flexible time range. Might be the last 5 days or the last 5 years depending on configuration and actual traffic.
What about adding the information "since [date]" , either instead or in addition of "all time"?

[yurihs]

That makes sense! Please have another look @stklcode.

[stklcode]

Great! I've added a minor CSS correction for alignment with the pre-existing tables.

Works fine on top of the current master. With #72 merged, $stats['visits'][0]['date'] is no longer correct, because this would display the overall totals from the database along with the oldest date from the display range.

Feel free to work around this, but I have to rework my PR anyway, so I can take care of that there.