Code style tweaks on `cli.js`.

* Use `split` to parse docopt comment instead of `substr`.
* Remove unused catch (line 47) that was only logging the error and propagating it; the uncaught exception already does this.
* Replace double quotes by single quotes.
* Remove all `cwd` instances since every path (except for `require` that needs an heading `./`) is already relative to `cwd`.
* Refactorize a too long ternary operator in a classic if/else.
* Code style for catch blocks; the `catch` goes on the same line as the `try` closing brace (like for an if/else), and there's a space before the catch opening paren.
* Remove `<src>` and `<dest>` undefined check since docopt already requires these parameters to be set (they must be surrounded by brackets in the usage string to make them optional).