absolute path as base directory (`-d` option)

The `-d` (`--dist`) option seems to work only with a path under the current directory (`process.cwd()`). Is there way to use an absolute path like `/tmp`?

```
$ npx gh-pages -V
2.1.1
$ pwd
/Users/okuryu/work/tests/test-20191002
$ env NODE_DEBUG=gh-pages npx gh-pages -d /tmp/docs/
ENOENT: no such file or directory, stat '/Users/okuryu/work/tests/test-20191002/tmp/docs/'
```
