Customize time format for graphs

**Proposal:**
Ability to customize the timestamp format beyond what's available in the dropdown currently.

**Current behavior:**
You must select from a variety of options, but they all include either:
a) the Y M D and time
b) just the time

**Desired behavior:**
The ability to just include variations of the Y, M, D combinations.

**Use case:**
Users who have daily data might not want to include hour minute second timestamps.

https://community.influxdata.com/t/custom-time-format-for-graphs-date-only/25937