Do not mutate the options object

Avoid mutating the options object as:

1. Consumers may reuse the given object elsewhere in their code;
2. [lazypipe](https://github.com/OverZealous/lazypipe) expects immutable arguments, it will throw an error when used with gulp-jscs at the moment.
Merge pull request #46 from UltCombo/options-arg

Do not mutate the options object