Let mongofixtures accept absolute paths to the fixture

Currently it will only accept relative paths, and will try to append a given absolute path to the cwd. This pull request will allow it to handle both.

Before:

```bash
./bin/mongofixtures db foo.js
# --> /Users/evdb/git/pow-mongodb-fixtures/foo.js
./bin/mongofixtures db /tmp/foo.js
# --> /Users/evdb/git/pow-mongodb-fixtures//tmp/foo.js
```

After:

```bash
./bin/mongofixtures db foo.js
# --> /Users/evdb/git/pow-mongodb-fixtures/foo.js
./bin/mongofixtures db /tmp/foo.js
# --> /tmp/foo.js
```