Tootgroup.py

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FIXED

- Media upload was broken due to change in Mastodon's media.url

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FIXED

- Retoots of direct messages do no longer show HTML-escaped special characters

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ADDED

- Read commandline flags with agparser
- Added --user flag to configure and use multiple accounts
- When sharing access to a group, `tootgroup.py` does no longer look for the last
time it wrote anything to the group to determine what has happened since. In
that case it persists timestamps and checks when the last run has happened to
search for any new toots or direct messages to process. This is needed because
any toots from other sources would cause `tootgroup.py` to miss notifications that
happened between it and tootgroup's last run. This feature writes the config
file to the disk every thime `tootgroup.py` is run and for that reason it is only
activated if a shared account requires it.

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FIXED

- Retoots of direct messages do now keep their linebreaks
- Removed hardcoded groupname

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ADDED

- Read and persist configuration with configparser
- Detect problems with configuration and get user input for corrections
- First-run/Install feature. Detect missing configuration and ask user for all
necessary setup details.

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CHANGED

- Retoot mode is no longer triggered by using a specific hastag, but by preceding
the mention with an Exclamation Mark like "!mastodon"
- Code refactoring and cleanup. `tootgroup.py` behaves now more like a real
commandline application.