Both

Please allow `'both'` as an alias to `['client', 'server']`. I realize that not entering any value is even shorter, but that solution is not explicit. TBH, I didn't realize it at first as it wasn't documented... this builds on #1774.

This would avoid having to write `both = ['client', 'server']` like [here](https://github.com/bline/meteor-locale/blob/8b27d61aea1aa737b49192c30492ecb5ae660acb/package.js), [here](https://github.com/BeDifferential/meteor-profile/blob/a87ef01b5cac68ff172ba0ecd4afe9837aa6eb8f/package.js), [here](https://github.com/mindreframer/meteorjs-stuff/blob/f4de16806497006bbe8f849d9a027be983d4bcab/github.com/alanning/meteor-roles/roles/package.js),  [etc](https://github.com/RolandStuder/jeizinen-meteor/blob/d8b8ffe5689fd65f24cedd09ec54df3ea1351933/package.js)...