Moves from optimist to minimist

I revamped the options parsing for the `cite` CLI tool.

* Switches out the (deprecated) [`optimist`](https://github.com/substack/node-optimist) to use its smaller, more predictable replacement, [`minimist`](https://github.com/substack/minimist).
* Aliases (short flags, e.g. `-t` and `-p`) are removed.
* The `--pretty` option is now removed. All output is now pretty-printed (with 2-space indents).
* All options are now passed to `Citation.find()`, not just whitelisted options.
* This means you can pass arbitrarily nested options over the command line. For example, `--dc_code.source=dc_code` will create an options object of `{dc_code: {source: 'dc_code'}}`.
* Updated the documentation to reflect all of this.

While all the tests pass, the tests don't really touch the CLI tool. I've done empirical testing on the CLI tool that confirms to me that I haven't broken anything, but I could be wrong.

Resolves the issues raised in #90.