caption_hashtags property for only-if evaluation

caption_hashtags is a list of all hashtags that are mentioned in the
Post's caption. It allows to easily filter Posts that have multiple
hashtags, and as such fixes #24.

Further, the documentation of --only-if has been completed by linking to
a description of the syntax in the Python documentation, and by linking
to a list of all defined properties with their meanings. So, this commit
also closes #42.

instaloader.py

@@ -275,6 +275,16 @@ class Post:
         elif "caption" in self._node:
             return self._node["caption"]
 
+    @property
+    def caption_hashtags(self) -> List[str]:
+        """List of all hashtags (without preceeding #) which occur in the Post's caption."""
+        if not self.caption:
+            return []
+        # This regular expression is from jStassen, adjusted to use Python's \w to support Unicode
+        # http://blog.jstassen.com/2016/03/code-regex-for-instagram-username-and-hashtags/
+        hashtag_regex = re.compile(r"(?:#)(\w(?:(?:\w|(?:\.(?!\.))){0,28}(?:\w))?)")
+        return re.findall(hashtag_regex, self.caption)
+
     @property
     def is_video(self) -> bool:
         """True if the Post is a video."""