0.3.0

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 funtions (encrypt/decrypt)
* Rework error messaging repeatable code (moved in utils)
* Reduce code duplication (requests session configuring)