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
 ^^^^^^^^^^^^^^
 

docs/basic-usage.rst

@@ -7,3 +7,9 @@ Basic Usage
 .. include:: ../README.rst
    :start-after: basic-usage-start
    :end-before: basic-usage-end
+
+.. (continuation of --only-if explanation)
+
+The given string is evaluated as a
+`Python boolean expression <https://docs.python.org/3/reference/expressions.html#boolean-operations>`__,
+where all occuring variables are properties of the :ref:`post-class`.