feat: support for custom templates

[hacdias]

Closes #125 (see for more context). This PR:

• adds -templates flag that can be used to provide a directory with templates. They override existing ones for users that want to customize how the pages look.
• added a template for 404 pages
• compressed template handling into serveHTML

[hacdias]

@abhinav just pinging you, in case you have a chance to look at this 😄 Also, thanks for merging the other PRs!

[abhinav]

Thanks for this and the other PRs, @hacdias, and thanks for waiting. I had to make some time to look over this. (The other PRs were a lot quicker to review. 😅)

I'm in favor of this change in general, and I appreciate the rigor that went into this. This is a good PR. I have only a couple minor suggestions. It should be good to merge after that.

Thanks!

[hacdias]

@abhinav thanks! I addressed your feedback, and also cleaned up a few things I saw along the way. Looking forward to see this merged. I found many packages like this one, but none did exactly what I wanted.

[abhinav]

No blockers. This is great! Thanks @hacdias!

I'm okay with this as-is, but if you'd like to make getTestTemplates take a testing.TB, I can wait a day.

[hacdias]

@abhinav thanks for the review! I applied the feedback, and I think it's ready to be merged.

[codecov]

Codecov Report

Merging #129 (acbf6db) into master (a068bd4) will decrease coverage by 4.04%.
The diff coverage is 65.71%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   80.14%   76.11%   -4.04%     
==========================================
  Files           3        3              
  Lines         136      180      +44     
==========================================
+ Hits          109      137      +28     
- Misses         23       39      +16     
  Partials        4        4              

Files	Coverage Δ
handler.go	94.91% <86.95%> (+1.16%)	⬆️
main.go	16.21% <25.00%> (+16.21%)	⬆️

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[abhinav]

Merged! Thanks, @hacdias