Configure prettierrc

[dapplion]

Brief aside, but we should be able to have prettier add ending commas in cases like this, to decrease the line diffs.

Originally posted by @sohkai in https://github.com/aragon/buidler-aragon/pull/90#discussion_r402933656

---

Add

"trailingComma": "es5"

To the .prettierrc file

Then run

node_modules/.bin/prettier --write "**/*.{ts,js}"

Warning: will generate a massive diff, do in a period of low development