Implement #22 - CLI options, -r(equire) <file>

Adds command-line argument support, using `getopt`.

```
filipe · ~/dev/boris/bin · ⭠ feature/cli-args $ ./boris --help
Usage: boris [options] [includes]
boris is a tiny REPL for PHP

Options:
  -h, --help     show this help message and exit
  -r, --require  a comma-separated list of files to require on startup
```

Regarding `require`, multiple files may be required at once:

```
$ boris -r hello.php
$ boris -r hello.php,foobar.php
$ boris -r hello.php -r foobar.php
$ boris -r hello.php --require foobar.php,foo.php,autoload.php -r bananas.php
```

You get the idea.

Cheers!