enforce use of catch() on un-returned promises

Use eslint-plugin-promise to enforce use of catch() on un-returned promises.

[petty]

multiple blank lines, lines at BOF and EOF

Add import plugin and test environment settings

Add eslint-plugin-import

https://github.com/benmosher/eslint-plugin-import#readme

Add test env jest / mocha

Included in @fiverr/fiverr:

• base: Just the set of rules
• es6: New ES support and rules
• react: React specific rules and eslint-plugin-react plugin

Need to be added explicitly:

• jest
• mocha