Variable arguments and falsy values

Adds ability to handle passing in variable arguments (ex. `chalk.green("it's", "over", 9000)`). This is done by checking if the array of arguments is greater than 1. If it is, then set `str` to an empty string, loop through each argument, add that argument to `str`, and add a space after the argument unless it's the last one. Basically I tried to mimic the way `console.log` works. Feel free to comment/correct if there's a better of doing this.

Also adds the ability to accept falsy values like 0, fixing issue #6.

In addition to that the readme has been updated to include passing in variable arguments.