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/css.go

@@ -0,0 +1,22 @@
+package html
+
+import (
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+func HasClassStrings(node *html.Node, classStrings ...string) (match bool) {
+	targetClasses := make(map[string]struct{}, len(classStrings))
+	for _, classString := range classStrings {
+		targetClasses[classString] = struct{}{}
+	}
+
+	classAttribute := Attribute(node, "class")
+	classes := strings.Fields(classAttribute)
+	for _, class := range classes {
+		delete(targetClasses, class)
+	}
+
+	return len(targetClasses) == 0
+}