Adds a new abstract `Input` interface for the manager to use different
input handlers with the same methods. The Nordigen class and a new JSON
parser descend from it. The previous csv parser will also eventually.
New converter for list input also added.
Issue #19
To improve the code organization, additional subfolders were added into
the pfbudget module.
The barebones of renewing the token and downloading transactions
directly from the banks through the Nordigen API were also added.
Issues #13, #15