feat: support -e as shortened flag for --exclude-namespaces

[doronkg]

What this PR does / why we need it

This PR adds support for -e as shortened flag for --exclude-namespaces.

$ kor all --exclude-namespaces ns1,ns2
$ kor all -e ns1,ns2

In additon, an updated output of kor --help was added to README.md.

PR Checklist

• [ ] This PR adds K8s exceptions (false positives)
• [x] This PR adds new code
• [ ] This PR includes test for any new code

Github Issue

None.

Notes for your reviewers

The disparity was found while working on #258. The -e flag was referenced in https://github.com/yonahd/kor/blob/main/pkg/filters/options.go#L116, but was missing in root.go.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 43.37%. Comparing base (da2b1fe) to head (85c77fb). Report is 1 commits behind head on main.

Files	Patch %	Lines
cmd/kor/root.go	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #261 +/- ## ======================================= Coverage 43.37% 43.37% ======================================= Files 58 58 Lines 2808 2808 ======================================= Hits 1218 1218 Misses 1400 1400 Partials 190 190 ```

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