Vue-i18n: Typescript support

[aom]

I suppose this a feature request just like any other.

Facebook's FlowType is supported using decls/. I'd like to see Typescript support. Two ways to accomplish this:

1) Add ie. types/ folder with d.ts files
2) Add types to https://github.com/typings/typings so they can be added as @types/vue-i18n

I'd prefer the first one as it's easier to maintain with other parts of the codebase.

Unfortunately I'm not fluent enough with Typescript yet to help much with contribution. However I got it working for me with this afternoon hack: https://github.com/aom/vue-i18n/tree/typescript-support

[aom]

Found one related discussion from StackOverflow which specifically uses this wonderful project as an example: http://stackoverflow.com/questions/42028790/vscode-and-vue-2-plugin-typescript-definitions

[aicest]

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/vue-i18n/index.d.ts

Use it via npm i -D @types/vue-i18n or yarn add -D @types/vue-i18n.