Initial value in minimum/maximum for RangeCumsum

[jishnub]

With this,

julia> minimum(RangeCumsum(2:1), init=10)
10

julia> maximum(RangeCumsum(2:1), init=0)
0

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.95%. Comparing base (4aa8264) to head (3e18e25). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #222 +/- ## ========================================== + Coverage 89.58% 89.95% +0.36% ========================================== Files 11 11 Lines 1844 1891 +47 ========================================== + Hits 1652 1701 +49 + Misses 192 190 -2 ```

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