feat: SlickVPN Support (#961)

- `internal/updater/html` package
- Add unit tests for slickvpn updating code
- Change shared html package to be more share-able
- Split html utilities in multiple files
- Fix processing .ovpn files with prefix space

Authored by @Rohaq 
Co-authored-by: Quentin McGaw <quentin.mcgaw@gmail.com>

internal/updater/openvpn/extract.go

@@ -76,6 +76,7 @@ func ExtractIPs(b []byte) (ips []net.IP, err error) {
 func extractRemoteHosts(content []byte, rejectIP, rejectDomain bool) (hosts []string) {
 	lines := strings.Split(string(content), "\n")
 	for _, line := range lines {
+		line = strings.TrimSpace(line)
 		if !strings.HasPrefix(line, "remote ") {
 			continue
 		}