Named query positional parameters aggregation

[chagui]

Description

This pull-request propose an implementation for #106. The proposal adds 2 new placeholders $* and $@, respectively raw aggregation and string aggregation. The difference being that the later will quote the argument values, removing the need to quote the argument (e.g. 'foo' instead of \'foo\').
I took the addition to add tests for subst_favorite_query_args covering both current and new placeholders.

Checklist

• I've added this contribution to the changelog.rst.
• I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@20dc99f). Click here to learn what that means.
The diff coverage is 65.51%.

@@            Coverage Diff            @@
##             master     #108   +/-   ##
=========================================
  Coverage          ?   29.08%           
=========================================
  Files             ?        6           
  Lines             ?     2486           
  Branches          ?        0           
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  Hits              ?      723           
  Misses            ?     1763           
  Partials          ?        0           

Impacted Files	Coverage Δ
pgspecial/help/commands.py	0.20% <ø> (ø)
pgspecial/main.py	38.18% <52.17%> (ø)
pgspecial/iocommands.py	41.86% <65.15%> (ø)
pgspecial/dbcommands.py	33.60% <66.73%> (ø)
pgspecial/namedqueries.py	92.00% <75.00%> (ø)
pgspecial/__init__.py	100.00% <100.00%> (ø)

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[amjith]

Looks good. Thanks for adding tests. 👍