Added support for group topics

[Anton2319]

This PR adds support for topic-structured groups, officially known as forums. This was requested in Issue #131. Legacy supergroup behaviour is unchanged; new logic is only applied when telegram_send detects that it is being configured to work inside a forum. Topic-separation works by replying to a "forum_topic_created" message. An additional configuration field was added to keep a permanent reference to this message, called "reply_to_message_id". If "reply_to_message_id" is present in the configuration, the bot will always reply to that message, which will result in a message appearing inside a topic. Examples of reworked logic are shown below:

Legacy groups:

Forums:

Forums (alternated topic):

New configuration:

Old configuration:

[renich]

interested in this one.