Add custom buffer constructor

[TheMadman]

Implements the ability to pass a custom memory buffer for use in the CMS struct.

Could be useful for e.g. mmap'd buffers for graceful restart, shared memory buffers for syncing between processes, etc.

Has appropriate documentation/warnings

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 52.17391% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 93.79%. Comparing base (4fd7e2a) to head (d76b55c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   96.88%   93.79%   -3.10%     
==========================================
  Files           2        2              
  Lines         257      274      +17     
==========================================
+ Hits          249      257       +8     
- Misses          8       17       +9     

Files	Coverage Δ
src/count_min_sketch.h	100.00% <ø> (ø)
src/count_min_sketch.c	93.30% <52.17%> (-3.32%)	:arrow_down: