Change package structure - enable plugin support

[pylipp]

WIP

• this is being solved by making financeager support Python plugins. It brings the opportunity to modularize the code base, and to enable extending the core package easily
• financeager version 0.23.0.0 was released, bringing plugin support

Current Situation

financeager can be used to communicate with a database on the local machine, or a database on a remote machine (which has a webservice installed).
Having both ways combined in a single package is powerful but has some smells. First of all it has to be reconsidered of how much value the 'remote' variant is at all.
When users want to use the 'local' variant,

• they still have to install dependencies relevant to the remote variant (requests, Flask)
• some package functionality is irrelevant (offline, httprequests, fflask, resources modules)

When users want to use the 'remote' variant,

• they have to configure financeager on the client side
• set up the webservice running behind an FCGI/WSGI

which might be too much of a hazzle already.

Now one major feature I'd like to introduce is a financeager client TUI (like e.g. tig or pudb). When used with the 'remote' variant however, the client would have to hold a copy of the remote database for performance/connectivity reasons, and sync this local copy periodically. This brings

• need for error-prone synchronization logic (re-inventing the wheel?)
• a local database which is already exactly what the local variant would do

Proposal

• the 'remote' variant is marked as deprecated
• the 'remote' variant implementation is split from the package
• the 'remote' variant implementation can be additionally installed and, by config, be selected for use
• the 'local' variant becomes the core of financeager

Then

• financeager can be extended by a TUI mode (see Non-CL user interface #50) which accesses the local database
• for synchronizing database files (if desired) users can
  • select a cloud storage service (e.g. Dropbox)
  • use version control, also in combination with a service, or a remote