Add path to metadata JSON

[csala]

If a table is added to Metadata passing a path, the path should be added to the metadata dictionary alongside its fields:

In [1]: from sdv import Metadata                                                                                                      

In [2]: metadata = Metadata()                                                                                                         

In [3]: metadata.add_table('customers', 'quickstart/customers.csv')                                                                   

In [4]: metadata.to_dict()['tables']['customers'].keys()                                                                              
Out[4]: dict_keys(['fields'])