adding convertArea function

[wnordmann]

Added in a convertArea function that follows the design of convertDistance
As discussed in issue - #817

• Use a meaningful title for the pull request. Include the name of the package modified.
• Have read How To Contribute.
• Run npm test at the sub modules where changes have occurred.
• Run npm run lint to ensure code style at the turf module level.

Submitting a new TurfJS Module.

• Overview description of proposed module.
• Include JSDocs with a basic example.
• Execute ./scripts/generate-readmes to create README.md.
• Add yourself to contributors in package.json using "Full Name <@github Username>".

[DenisCarriere]

👍 great work @wnordmann