Allow using a custom KV store to keep consistent JSON ordering

[alecsammon]

See #50 for full discussion

Fixes: #49

Allows for overwriting the default map[string]interface{} to allow for consistent ordering in the generated JSON.

[coveralls]

coverage: 89.167% (+0.5%) from 88.646%
when pulling 3ffae7a on alecsammon:custom_kvstore
into 60a4260 on liip:master.

[coveralls]

coverage: 89.167% (+0.5%) from 88.646%
when pulling 71dfc70 on alecsammon:custom_kvstore
into 60a4260 on liip:master.

[coveralls]

coverage: 89.167% (+0.5%) from 88.646%
when pulling 71dfc70 on alecsammon:custom_kvstore
into 60a4260 on liip:master.

[alecsammon]

Performance:
Before change:

1. BenchmarkModelsMarshaller_Marshal-8 78291 15360 ns/op
2. BenchmarkModelsMarshaller_Marshal-8 78034 15334 ns/op
3. BenchmarkModelsMarshaller_Marshal-8 77398 15560 ns/op
4. BenchmarkModelsMarshaller_Marshal-8 78068 15211 ns/op
5. BenchmarkModelsMarshaller_Marshal-8 78171 15280 ns/op

With change

1. BenchmarkModelsMarshaller_Marshal-8 77037 15457 ns/op
2. BenchmarkModelsMarshaller_Marshal-8 77643 15623 ns/op
3. BenchmarkModelsMarshaller_Marshal-8 74809 15513 ns/op
4. BenchmarkModelsMarshaller_Marshal-8 76140 15480 ns/op
5. BenchmarkModelsMarshaller_Marshal-8 77620 15347 ns/op

Average before: 15349 ns/op
Average after: 15484 ns/op

[mweibel]

👍 looks good to me

[mweibel]

thanks for the contributions @alecsammon - I released those as v2.0.0-beta.2. Unless I get issues reported in the next few weeks I'll make a stable release afterwards.

[alecsammon]

@mweibel - thank you so much!