Feature request: anchoring suffix for hours
#### Code Sample, a copy-pastable example if possible

```python
# It is possible to use anchor suffix for weeks
off1 = pd.tseries.frequencies.to_offset('1W-SUN')

# The anchor does effect when using period_range() or creating a period
ts_start = pd.Timestamp('2020-03-25 10:00') # ts_start is a Wednesday

p1 = pd.Period(ts_start, freq=off1)
# Anchor is working: p1 with weekly frequency starts a Monday
p1.start_time
>>> Out: Timestamp('2020-03-23 00:00:00')

# But it doesn't work with '4H' frequency
off2 = pd.tseries.frequencies.to_offset('4H')

# No anchor (expecting one with reference being midnight)
off2.is_anchored()
>>> False

p2 = pd.Period(ts_start, freq=off2)

# Expecting '2020-03-25 08:00:00'
p2.start_time
>>> Timestamp('2020-03-25 10:00:00')

# Trying syntax similar to weekly anchoring
off3 = pd.tseries.frequencies.to_offset('4H-0')
>>> ValueError: Invalid frequency: 4H-0
```
#### Problem description
Please, can anchoring suffix be made available for hours, using midnight as the reference point?

Thanks for your feebdack,
Have a good day,
Bests,