Add --denote-weekends (#54)

[willkg]

This makes it easier to line up weekends and weekend lull in numbers.

Fixes #54

$ supersearchfacet --denote-weekends --_histogram.date=product
product
 histogram_date | -- | Fenix | Firefox | Focus | MozillaVPN | ReferenceBrowser | Thunderbird | total  
----------------|----|-------|---------|-------|------------|------------------|-------------|--------
 2023-10-03     | 0  | 49585 | 62097   | 938   | 1          | 1                | 18202       | 130824 
 2023-10-04     | 0  | 49559 | 60180   | 907   | 0          | 0                | 18888       | 129534 
 2023-10-05     | 0  | 48276 | 59486   | 871   | 1          | 0                | 18532       | 127166 
 2023-10-06     | 0  | 49019 | 55451   | 957   | 0          | 0                | 16913       | 122340 
 2023-10-07 **  | 0  | 49089 | 41220   | 931   | 0          | 0                | 7858        | 99098  
 2023-10-08 **  | 0  | 50284 | 38363   | 895   | 1          | 0                | 7040        | 96583  
 2023-10-09     | 0  | 49355 | 62530   | 784   | 0          | 0                | 18934       | 131603