Allow user to pass custom epicMiddleware to epicManager

[Lucklyric]

It is common to have an epicMiddleware that includes dependencies.
In the current observable extension, it creates a default epicMiddleware which will not work for users with custom epicMiddleware.
In this PR, I simply added an argument to getObservableExtension to allow user to pass custom epicMiddleware to epicManger in the extension.

export function getObservableExtension(_epicMiddleware: EpicMiddleware<any>): IExtension;

[msftclas]

All CLA requirements met.

[olegshilov]

any news here?

[LFFATE]

well. it's also will solve my problem #145
So waiting for review this PR