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.

docs/as-module.rst

@@ -16,6 +16,8 @@ Introduction
    :members:
    :undoc-members:
 
+.. _post-class:
+
 ``Post`` Class
 ^^^^^^^^^^^^^^