ENH: Add pytest cli options and ini configurations

[Sheila-nk]

• Implemented the pytest_addoption hook function to register custom cli options and ini configurations
• Updated pyproject.toml file with dependencies and plugin entry-point
• You can now use cli command --doctest-scpdt when running pytest to enable doctesting through the scpdt tool

TODO:

• Bug: doctests for *.rst files are currently being collected twice on running:
  python -m pytest scpdt/tests --doctest-scpdt --doctest-glob='*.rst'
• Discuss custom cli options and ini configurations for scpdt

Closes #67

[Sheila-nk]

@ev-br
So I added the --doctest-scpdt cli option that is meant to enable doctests using the scpdt tool and --doctest-only cli option that is meant to ensure only doctests are carried out.
I also added the doctest_options ini configuration that specifies the default doctest options for the scpdt tool.
I don't think I have exhausted the cli options and configurations, that's why I made this a draft PR instead so we can have a discussion about amending or adding to the list.
My main point of confusion is what attributes and configurations we should set using the hooks and what we should let DTConfig take care of.

[Sheila-nk]

After changing the pytest_configure hook to override doctest functions and classes when the cli command --doctest-scpdt is used or when the doctest_scpdt ini option is True, I don't think there's need to implement the pytest_collect_files hook since the test collectors have already been overridden.

[Sheila-nk]

I don't think there's need to implement the pytest_collect_files hook since the test collectors have already been overridden.

I take this back, without the pytest_collect_file hook, all doctests are not being collected. Lol!

[Sheila-nk]

I believe we can close this PR now since it is a duplicate of @ev-br's merged PR: #128 which implements the pytest_addoption hook function to register custom cli options.
Also, all doctests are being collected without the implementation of the pytest_collect_file hook.