Allow transform of Transformers to return promise

[azu]

It allow Transformer#transform to work as async.

I think that it is needed to implemenet async tranform like stylelint #118 and textlint.
These tools has only async linting(transform) function.

• https://github.com/textlint/textlint/blob/master/docs/use-as-modules.md#core textlint.lintText() return Promise<TextLintResult>
• http://stylelint.io/user-guide/node-api/ stylelint.lint() reutn Promise.

P.S. I work on adding textlint to astexplorer which will be fork version. textlint#1
Async transform was needed to support textlint's transform.

[fkling]

Sorry for not replying earlier, this looks fine to me.