Append relative base href to siteURL

[IndrekHaav]

Favicon extraction fails when the website's base href is set to a relative URL. For example, printables.com:

$ wget -q -O - https://printables.com | grep 'base href'
  <base href="/">

In this case fetching the icons fails because only the relative URL is used:

$ PORT=8000 bin/iconserver
iconserver v3.18.0 () (go1.21.1) - https://github.com/mat/besticon
server: 2024/05/03 14:23:12.407321 Starting server on 0.0.0.0:8000...
http:  2024/05/03 14:23:14.890857 GET http://printables.com 200 357.50ms -1
http:  2024/05/03 14:23:14.893386 Error: GET http:///favicon.ico Get "http:///favicon.ico": http: no Host in request URL 0.08ms
http:  2024/05/03 14:23:14.893609 Error: GET http:///assets/favicons/favicon-16x16.png Get "http:///assets/favicons/favicon-16x16.png": http: no Host in request URL 0.02ms
http:  2024/05/03 14:23:14.893601 Error: GET http:///assets/favicons/apple-touch-icon.png Get "http:///assets/favicons/apple-touch-icon.png": http: no Host in request URL 0.05ms
http:  2024/05/03 14:23:14.893640 Error: GET http:///apple-touch-icon-precomposed.png Get "http:///apple-touch-icon-precomposed.png": http: no Host in request URL 0.03ms
http:  2024/05/03 14:23:14.893713 Error: GET http:///apple-touch-icon.png Get "http:///apple-touch-icon.png": http: no Host in request URL 0.01ms
http:  2024/05/03 14:23:14.893705 Error: GET http:///assets/favicons/favicon-32x32.png Get "http:///assets/favicons/favicon-32x32.png": http: no Host in request URL 0.02ms
server: 2024/05/03 14:23:14.895058 GET /icons?url=printables.com 404 "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:125.0) Gecko/20100101 Firefox/125.0"  393.43ms 2780
^C

Note all the "no Host in request URL" errors. This PR should fix this by appending relative base href values to siteURL:

$ PORT=8000 bin/iconserver
iconserver v3.18.0 () (go1.21.1) - https://github.com/mat/besticon
server: 2024/05/03 14:23:48.280063 Starting server on 0.0.0.0:8000...
http:  2024/05/03 14:23:53.088582 GET http://printables.com 200 353.14ms -1
http:  2024/05/03 14:23:53.111309 GET https://www.printables.com/assets/favicons/apple-touch-icon.png 200 17.58ms 1867
http:  2024/05/03 14:23:53.112752 GET https://www.printables.com/apple-touch-icon.png 200 18.87ms -1
http:  2024/05/03 14:23:53.113204 GET https://www.printables.com/apple-touch-icon-precomposed.png 200 19.53ms -1
http:  2024/05/03 14:23:53.113392 GET https://www.printables.com/favicon.ico 200 19.62ms -1
http:  2024/05/03 14:23:53.113405 GET https://www.printables.com/assets/favicons/favicon-32x32.png 200 19.61ms 781
http:  2024/05/03 14:23:53.113232 GET https://www.printables.com/assets/favicons/favicon-16x16.png 200 19.68ms 616
server: 2024/05/03 14:23:53.116060 GET /icons?url=printables.com 200 "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:125.0) Gecko/20100101 Firefox/125.0"  384.42ms 5339
^C

Apologies in advance if the code is not up to best practices, this is literally my first time writing Golang.

[mat]

Thank you for your contribution and the well written PR description, @IndrekHaav