Expose and handle --config argument

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

This PR builds upon #244, to allow explicitly specifying the location of pyproject.toml, which can be useful for projects passing a root directory different than . where pyproject.toml is stored in the specified root directory.

[codecov-commenter]

Codecov Report

Merging #245 (d631eca) into main (efcfc03) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #245     +/-   ##
=======================================
- Coverage   95.6%   95.6%   -0.1%     
=======================================
  Files         32      32             
  Lines        904     899      -5     
  Branches     198     189      -9     
=======================================
- Hits         865     860      -5     
  Misses        26      26             
  Partials      13      13             

Impacted Files	Coverage Δ
deptry/cli.py	93.3% <100.0%> (-0.2%)	⬇️
deptry/config.py	100.0% <100.0%> (ø)
deptry/core.py	97.6% <100.0%> (+<0.1%)	⬆️
deptry/dependency_getter/base.py	95.4% <100.0%> (+0.4%)	⬆️
deptry/dependency_getter/pdm.py	100.0% <100.0%> (ø)
deptry/dependency_getter/pep_621.py	92.5% <100.0%> (-0.4%)	⬇️
deptry/dependency_getter/poetry.py	100.0% <100.0%> (ø)
deptry/dependency_specification_detector.py	100.0% <100.0%> (ø)
deptry/utils.py	85.7% <100.0%> (-1.0%)	⬇️

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