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/provider/slickvpn/provider.go

@@ -0,0 +1,33 @@
+package slickvpn
+
+import (
+	"math/rand"
+	"net/http"
+
+	"github.com/qdm12/gluetun/internal/constants/providers"
+	"github.com/qdm12/gluetun/internal/provider/common"
+	"github.com/qdm12/gluetun/internal/provider/slickvpn/updater"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+type Provider struct {
+	storage    common.Storage
+	randSource rand.Source
+	utils.NoPortForwarder
+	common.Fetcher
+}
+
+func New(storage common.Storage, randSource rand.Source,
+	client *http.Client, updaterWarner common.Warner,
+	parallelResolver common.ParallelResolver) *Provider {
+	return &Provider{
+		storage:         storage,
+		randSource:      randSource,
+		NoPortForwarder: utils.NewNoPortForwarding(providers.SlickVPN),
+		Fetcher:         updater.New(client, updaterWarner, parallelResolver),
+	}
+}
+
+func (p *Provider) Name() string {
+	return providers.SlickVPN
+}