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/updater/updater.go

@@ -0,0 +1,22 @@
+package updater
+
+import (
+	"net/http"
+
+	"github.com/qdm12/gluetun/internal/provider/common"
+)
+
+type Updater struct {
+	client           *http.Client
+	parallelResolver common.ParallelResolver
+	warner           common.Warner
+}
+
+func New(client *http.Client, warner common.Warner,
+	parallelResolver common.ParallelResolver) *Updater {
+	return &Updater{
+		client:           client,
+		parallelResolver: parallelResolver,
+		warner:           warner,
+	}
+}