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/html/attribute.go

@@ -0,0 +1,12 @@
+package html
+
+import "golang.org/x/net/html"
+
+func Attribute(node *html.Node, key string) (value string) {
+	for _, attribute := range node.Attr {
+		if attribute.Key == key {
+			return attribute.Val
+		}
+	}
+	return ""
+}