Allow column renaming

Allow to rename MongoDB's attributes during the import.

In order to implement this feature I took some commits from @andrewjshults' [*dot-syntax* branch](https://github.com/andrewjshults/mosql/tree/dot-syntax).

This pull request introduces the new configuration format described inside of issue #4 but stays backward compatible with the current one.

It will be possible to mix the two configuration formats in this way:
```
   - (destination):
     :source: (source)
     :type: (type)
   - (source & destination): (type)
```