[BUG 🐞] Handling of missing information

[Bibo-Joshi]

Hi! Nice project you have here, really like it.

Describe the bug
I tried to run it on the rss feed of a tumblr blog and noticed that currently exceptions are raised when the RSS feed doesn't provide some of the info expected by tg-rss. More precisely, for me the following lines raised exceptions:

telegram-rss/telegram_rss/feed/updater.py

Lines 73 to 74 in de96efd

	self.feed_config.etag = raw_feed.etag
	self.feed_config.modified = raw_feed.modified

Both lines raise

File "/path/to/venv/lib/python3.8/site-packages/feedparser/util.py", line 158, in __getattr__
    raise AttributeError("object has no attribute '%s'" % key)
AttributeError: object has no attribute 'etag/modified'

telegram-rss/telegram_rss/feed/entry.py

Line 66 in de96efd

author=item["author"],

This raises

File "/path/to/venv/lib/python3.8/site-packages/feedparser/util.py", line 113, in __getitem__
    return dict.__getitem__(self, key)
KeyError: 'author'

When I comment the first two lines and hard-code an author, everything works fine so far.

To Reproduce
Steps to reproduce the behavior:

1. Run tg-rss on https://popsci.tumblr.com/rss

Desktop (please complete the following information):

• Python Version: 3.8.3
• tg-rss Version: 0.8.1

Additional context
As a side note, it would be neat if entries with images would be send as picture/media group with a caption. currently the picture is just shown via web page preview. I see that there is some logic for handling images in the code, but it's not used in send_message