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/connection.go

@@ -0,0 +1,13 @@
+package slickvpn
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+func (p *Provider) GetConnection(selection settings.ServerSelection) (
+	connection models.Connection, err error) {
+	defaults := utils.NewConnectionDefaults(0, 443, 0) //nolint:gomnd
+	return utils.GetConnection(p.Name(), p.storage, selection, defaults, p.randSource)
+}