Add support for `del` keyword for overridden functions

[sshishov]

Currently if you override the function of any framework or any library and you are not using all parameters, then the parameters which are not used are identified as unused. It is good practice while overriding function/method to keep the signature of it intact. Otherwise you can get potentially some issues with next releases etc.

Description

Adding support for del operation inside the function. The keyword del will mark the variable as used now and we can safely implement the suggested workaround.

Related Issue

This PR is addressing the issue #278

Checklist:

• [x] I have updated the documentation in the README.md file or my changes don't require an update.
• [x] I have added an entry in CHANGELOG.md.
• [x] I have added or adapted tests to cover my changes.
• [x] I have run tox -e fix-style to format my code and checked the result with tox -e style.

[codecov-commenter]

Codecov Report

Merging #279 (4c2ce60) into master (bbef855) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #279   +/-   ##
=======================================
  Coverage   99.37%   99.37%           
=======================================
  Files          18       18           
  Lines         635      635           
=======================================
  Hits          631      631           
  Misses          4        4           

Impacted Files	Coverage Δ
vulture/core.py	99.37% <ø> (ø)

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[sshishov]

Going to update the README and rebase from latest master.

[sshishov]

@jendrikseipp I have updated the README file. Could you please check? If you have better wording, please provide here, I will kindly make the changes.

[jendrikseipp]

Thanks for the revisions!