Interface formatting conventions

[reuveniy]

Support tslint interface rule

Description

Some projects have adopted https://palantir.github.io/tslint/rules/interface-name/ tslint interface rule making all interfaces starting with a captial I. graphqlgen does not comply with that and this proposal is to make it configurable so code generation will emit tslint friendly output.

Additional context

Implementation wide it should be pretty strait forward to add this configuraiton to the graphqlgen.yml file, read it and delegate it to the code gen template functions.

[jasonkuhrt]

@reuveniy what do you think if aside from there being a config it defaulted to whatever tslint had configured. Ideally the developer shouldn't have to manually configure this. They already did so once with tslint, after all.