Pbincli

New:
* Added validation of PrivateBin instance URL - 18 (it must contain trailing slash because POST is used)
* URL shortener support with various supported services - 19
* Shortener configuration, certificate validation and insecure warning settings can be configured in config file or via env

Changed:
* Restructured some parts of code by splitting big code chunks in functions (encrypt/decrypt)
* Rework error messaging repeatable code (moved in utils)
* Reduce code duplication (requests session configuring)

Fix swiped options booleans in v2 metadata for "open discussion" and "burn after reading" fields.

That version introduces paste version 2 support, which added in PrivateBin 1.3.

**License** was changed to **MIT**.
Added readme in RST (restructured text) format.
Whole code was refactored, all encoding/decoding code was moved to `format.py`.
Implemented instance paste version support checker (elrido).
Added v2 paste version support. New requirement - base58 package (elrido).
Reworked logic when no text would be sent, added flags to disable text including in paste.

In `api.py` was added SSL certificate validation disabling code, to bypass errors when server use self-signed certificate.