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!

[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.

[hacdias]

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

[codecov[bot]]

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%)	:arrow_up:
main.go	16.21% <25.00%> (+16.21%)	:arrow_up:

:mega: Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[abhinav]

Merged! Thanks, @hacdias