Updated for Swift 4

NoteSpeller.swift

struct NoteSpeller {
    static let wordURL = "https://raw.githubusercontent.com/dwyl/english-words/master/words.txt"
    static let delimiter: Character = "\n"
    static let notesNames: [Character] = ["a", "b", "c", "d", "e", "f", "g"]
    
    static func getWords(includeH: Bool = false) -> [String] {
        var notes = notesNames
        
        if includeH {
            notes.append("h")
        }
        
        guard let url = URL(string: wordURL),
            let file = try? String(contentsOf: url) else {
                return []
        }
        
        let words = file.split(separator: delimiter).map { String($0) }
        
        let result = words.filter { word in
            for character in word {
                if !notes.contains(character) {
                    return false
                }
            }
            
            return true
        }
        
        return result
    }
}

NoteSpeller.getWords()