Use minimist instead of optimist

* I replaced [optimist](https://github.com/substack/node-optimist) with [minimist](https://github.com/substack/minimist). This module doesn't use [a lot of optimist's methods](https://github.com/substack/node-optimist#methods).
* I removed unnecessary modules and variables.
* I fixed .npmignore. `node_modules` is ignored by default.