Add isopen method

[omus]

Fixes: #146

[codecov]

Codecov Report

Merging #147 (81fb5eb) into master (97146a9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   91.77%   91.78%           
=======================================
  Files          12       12           
  Lines        1180     1181    +1     
=======================================
+ Hits         1083     1084    +1     
  Misses         97       97           

Impacted Files	Coverage Δ
src/buffer.jl	100.00% <100.00%> (ø)

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[omus]

Bump @rofinn