Output

I would like to add an `--output` flag to the compiler along with the ability to compile multiple files at once, for the following reasons:

* **Familiarity**. Most compilers I've seen have this option. It's natural to try to use `-o` for outputting files. `coffee-script`, `rustc`, `gcc`, `nimrod`, `javac`, to list some examples
* **Ease of use inside terminal**. Say you are just wanting to compile a three odd files and don't want to use a build system yet. `wisp a.wisp b.wisp c.wisp -o ./` is a lot easier to type than repeatedly pushing the up arrow to retrieve the last three commands
* **Simplicity** It's a lot less likely to make typos like " > test.jjs" if the compiler handles it
* **CoffeeScriptRedux does have a `output` flag** https://github.com/michaelficarra/CoffeeScriptRedux/blob/master/src/cli.coffee#L148
* It does not really add much code to the compiler, just 12 lines.