fix: add watch mode

[josephktcheung]

This is to fix #45.

Added watch flag to enable watch mode.

If watch mode is toggled on:

1. the cli won't exit if error occurs, only prints error.
2. every time a file inside a data source directory is saved:
   1. it will clear all data sources' files (e.g. transpiled code / node_modules files) in require.cache. What that means is that next time the server loads, it will not load it from the cache, but read the file again. (Inspired by this article)
   2. On changes to the file we reattach the http server listener from old Express app to the new one. (Inspired by this article)