Convert code to Typescript

[ajbogh]

Is your feature request related to a problem? Please describe.

#138 provided a vote to convert to Typescript. The end result was 16 for and 7 against. We left the issue open for a little over 2 months to get all comments and votes.

Describe the solution you'd like

For this issue we will convert the codebase to Typescript.

• Ensure types are exported correctly
• Fix type errors in the code
• Set up tslint or eslint+typescript
• Investigate removing Babel and Webpack since tsc should handle compilation

[iChaosren]

Might help to get started: https://github.com/bcherny/flow-to-typescript