Make file extension configurable

[ajafff]

TypeScript parses and handles files different based on their extension:

• .ts -> regular TypeScript
• .tsx -> TypeScript with JSX, parses classic type assertions (<T>obj) as JSX elements
• .js -> regular JavaScript, uses JSDoc to annotate types
• .jsx -> JavaScript with JSX

It would be nice if the extension the editor uses was configurable to allow testing the differences between them.

[agentcooper]

After #35 it is .ts by default and .tsx if you turn on JSX option.