Add CatchAll functionality

[rybnico]

Description

This pull request is based on the closed PR #161

With the author's permission, I have incorporated the catch-all functionality into a new PR.

The catch-all functionality can be used especially when using error-pages in a Kubernetes project to - independent of the requested URL - always deliver an error page according to the X-Code HTTP header.

Since this PR has been closed before, please let me know what else is needed for this PR to be merged this time.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made changes in the CHANGELOG.md file

[tarampampam]

Looks pretty simple and cool! Did you test it on your side well?

[codecov]

Codecov Report

Merging #217 (bb70ad0) into master (ecf1359) will decrease coverage by 0.27%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
- Coverage   48.40%   48.13%   -0.27%     
==========================================
  Files          32       32              
  Lines        1250     1257       +7     
==========================================
  Hits          605      605              
- Misses        607      614       +7     
  Partials       38       38              

Files Changed	Coverage Δ
internal/cli/serve/command.go	0.00% <0.00%> (ø)
internal/env/env.go	100.00% <ø> (ø)
internal/http/server.go	0.00% <0.00%> (ø)

[rybnico]

Yes, I have tested it successfully. I created a Docker image containing the PR and pushed it to nrybnikar/error-pages:catch-all if you want to test it.
I rolled it out to our K8s cluster and also successfully tested the intended use, i.e. as a default-backend in an Ingress object.

[tarampampam]

You've done a tremendous job! Thank you!

[rybnico]

Thank you @tarampampam!
The credit should go to @chrisamti since he was the original author and I created this PR based solely on his work.