Automatically detect pyproject.toml

[econchick]

Hey, I just made a Pull Request!

Description

This addresses #36 - if a pyproject.toml exists for the project, interrogate will automatically pick up configured values and set them as the CLI options' defaults.

Motivation and Context

Have you tested this? If so, how?

Checklist for PR author(s)

• [x] Changes are covered by unit tests (no major decrease in code coverage %).
• [x] All tests pass.
• [x] Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example :smile:).
• [x] Updates to documentation:
  • [x] Document any relevant additions/changes in README.rst.
  • [x] Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • [x] Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note

Read configuration from ``pyproject.toml`` by default 

[codecov[bot]]

Codecov Report

Merging #42 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files           7        7           
  Lines         592      596    +4     
  Branches      125      126    +1     
=======================================
+ Hits          589      593    +4     
  Misses          1        1           
  Partials        2        2           

Impacted Files	Coverage Δ
src/interrogate/cli.py	100.00% <100.00%> (ø)
src/interrogate/config.py	100.00% <100.00%> (ø)

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