Add ability to pass in an array of vals instead of as separate params

- Fixed so `del` now accepts an array of keys rather than just lots of
params (should work for other commands too)
- Added test cases for the `del` command, testing both multiple params
and one array param
- Updated `.gitignore` to include `node_modules`
The way I've been handling this before now is by defining small wrapper methods around commands that accept multiple keys. See [the implementation of mget](https://github.com/mjackson/then-redis/blob/master/modules/Client.js#L137-L142) for an example. That way we only do the extra work on commands that may need it instead of all of them.