Lots of tests

NEW

• Added support for Imgur gallery images
• Better markdown escaping (Integrated python-telegram-bot's markdown escaping)
• Added docs. Documentation now lives at https://fabiosangregorio.github.io/telereddit/

BUGFIXES

• Fixed vreddit service bug
• Fixed bug where urls outside of reddit containing a r/ would be treated as subreddits.
• Fix bug where the exception sent as a message was not the one originally thrown

IMPROVEMENTS

• Refactored services for better exception handling in linker
• Refactored file structure
• Added lots of tests
• Added Design by Contract
• Added static type checking with mypy
• Added linting
• Added docstrings and docstrings linting