Refactor human interval using node-numbered package

[sampathBlam]

Refactor human interval using numbered package

• Supports numbers written as english words
• Supports negative numbers
• Supports hyphenated words
• Added more tests to verify the functionality
• Updated README.md

Closes #9 Updated after pulling changes from #19

[simison]

I've rebased this at https://github.com/agenda/human-interval/pull/37/