Add handling of negative indices to getitem

[pourhouse]

This will handle cases of ringbuf[-<n>], which were failing when self._arr is not full.

This may address the needs of your Issue #5, but I suspect it will not handle accesses when the index selector is an array_like with negative items.

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 0.08%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   98.49%   98.58%   +0.08%     
==========================================
  Files           3        3              
  Lines         266      282      +16     
==========================================
+ Hits          262      278      +16     
  Misses          4        4

Impacted Files	Coverage Δ
tests.py	98.76% <100%> (+0.1%)	:arrow_up:
numpy_ringbuffer/__init__.py	98.31% <100%> (+0.04%)	:arrow_up:

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