Command line interface

`nebu` command for the terminal.

```sh
Options:
-o output directory/filename
-m sourcemap option
-x ignored file patterns
```

When `-o` is unspecified, the result is printed to the console (if compiling a directory, we exit early).

The `-m` option can be `inline`, `both`, or no value (which saves sourcemaps to `.map` files alongside the compiled sources). When unspecified, sourcemaps are discarded.