Exclude generate proto code files in coverage

[alpe]

Resolves #298

With this patch we just ignore the Go files generated by protobuf and some test files.

Most of this is copied from https://github.com/cosmos/cosmos-sdk/blob/master/.codecov.yml . Thanks 💐

[codecov]

Codecov Report

Merging #320 (3e52ef6) into master (8ee7038) will increase coverage by 51.86%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #320       +/-   ##
===========================================
+ Coverage   17.65%   69.51%   +51.86%     
===========================================
  Files          35       28        -7     
  Lines       10592     2316     -8276     
===========================================
- Hits         1870     1610      -260     
+ Misses       8607      598     -8009     
+ Partials      115      108        -7     

Impacted Files	Coverage Δ
x/wasm/internal/types/query.pb.go
x/wasm/internal/types/types.pb.go
x/wasm/internal/types/query.pb.gw.go
x/wasm/internal/keeper/test_common.go
x/wasm/internal/types/proposal.pb.go
x/wasm/internal/types/genesis.pb.go
x/wasm/internal/types/msg.pb.go
x/wasm/internal/keeper/keeper.go	90.30% <0.00%> (+0.60%)	⬆️
x/wasm/internal/types/codec.go	45.83% <0.00%> (+0.67%)	⬆️

[orkunkl]

LGTM