Incorrect set of characters allowed for bare keys

[x-hgg-x]

Describe the bug
The set of characters allowed for bare keys is incorrect.

To Reproduce

type s struct {
	Name string `toml:"Name-àéù"`
	Num  int    `toml:"Num-𝟘"`
}

t, _ := toml.Marshal(s{"ok", 0})
fmt.Println(string(t))

Output:

Name-àéù = "ok"
Num-𝟘 = 0

Expected behavior
Expected output:

"Name-àéù" = "ok"
"Num-𝟘" = 0

Versions

• go-toml: last version of master (9ccd9bb)
• go: 1.14
• operating system: Linux

Additional context
None.