Support typescript

[pbadenski]

At the moment jp-babel doesn't work with TypeScript since babel does not by default include ".ts" in supported filename extensions (https://babeljs.io/docs/en/babel-core#default_extensions).

Additionally babel uses filename extensions to decide which presets should be enabled for a file and that was also not working.

We include to additional flags for jp-babel:

• extensions - user can override file extensions to be transpiled by babel (this is passed down to @babel/register)
• notebookLanguage - user can decide what language should be used to transpile notebook cells (instead of javascript which is the default)

This pull request combines all changes we did for our project to make jp-babel work.

This should also work for any other languages that babel can transpile, which are not enabled in babel by default.

//cc @robh1

[n-riesco]

@pbadenski Thank your for all the work on this PR and this review. But, since setting allExtensions to true seems to do the trick, I prefer this option over having more code to maintain.

I'd certainly welcome a PR to document your use of jp-babel and Typescript.