Ability to back up a subset of data by time

__Proposal:__
(Reported in Community Slack)

`influx backup` backs up an entire bucket every time. It'd be nice if it supported backing up a subset of data by time-range.

__Current behavior:__
`influx backup` exposes a single parameter for `--bucket-id`. It backs up the entirety of the bucket every time. This causes the size of backup files to be ever-increasing, and the matching restore times to also grow.

__Desired behavior:__
`influx backup` should be extended to have optional `--start` and `--end` parameters, like 1.x `influxd backup` did. These params would limit the scope of the backup.

Start at the `influx backup` CLI entry point and inspect how the existing `--bucket-id` option defines the backup scope. Trace the current bucket-wide behavior, then verify that optional `--start` and `--end` parameters limit the backed-up time range as described, reducing the data and restore scope.

Written by the indexing model from the issue text.