Remove HTML tags from content.

Do this as early as processing the entry so later steps can count
on it (esp. when counting characters)

Also add a new requirement: beautifulsoup4.

setup.py

@@ -14,6 +14,10 @@ setup(
     description='Connect an RSS Feed to Mastodon',
     long_description=long_description,
     long_description_content_type="text/markdown",
-    install_requires=['feedparser', 'mastodon.py', 'python-dateutil', 'pyyaml'],
+    install_requires=['beautifulsoup4',
+                      'feedparser',
+                      'mastodon.py',
+                      'python-dateutil',
+                      'pyyaml'],
     entry_points={'console_scripts': ['feediverse = feediverse:main']}
 )