Add custom message handler option

[alpe]

Resolves #327

[codecov]

Codecov Report

Merging #402 (2ac2b1e) into master (0f46dee) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   55.21%   55.43%   +0.21%     
==========================================
  Files          39       39              
  Lines        4015     4010       -5     
==========================================
+ Hits         2217     2223       +6     
+ Misses       1618     1608      -10     
+ Partials      180      179       -1     

Impacted Files	Coverage Δ
x/wasm/internal/keeper/options.go	100.00% <100.00%> (+100.00%)	⬆️
x/wasm/internal/types/codec.go	38.46% <0.00%> (-2.72%)	⬇️
x/wasm/internal/keeper/keeper.go	88.91% <0.00%> (+0.83%)	⬆️

[ethanfrey]

Nice one. Just exposing this with the options. Such a great design