Add singleton support for ease of use

[MJacred]

copied from my README.md changes:

Use the singleton to ease usage.

bundle := i18n.NewBundle(language.English)
bundle.RegisterUnmarshalFunc("toml", toml.Unmarshal)
bundle.MustParseMessageFileBytes([]byte(`
HelloCookie = "Hello Cookie!"

[Cookies]
one = "I have {{.PluralCount}} cookie!"
other = "I have {{.PluralCount}} cookies!"

[CookiesX]
other = "I have {{.X}} cookies!"

[CookiesX2]
one = "I have {{.PluralCount}} {{.X}} cookie!"
other = "I have {{.PluralCount}} {{.X}} cookies!"

[CookiesABC]
other = "I have {{.A}} cookies in my {{.B}} back at {{.C}}!"

[CookiesABC2]
other = "I have {{.PluralCount}} {{.A}} cookies in my {{.B}} back at {{.C}}!"
`), "en.toml")

localizer := i18n.NewLocalizer(bundle, "en-US")
i18n.SetLocalizerInstance(localizer)
i18n.SetABCParams([]string{"A", "B", "C"})

i18n.Localize("HelloCookie") // "Hello Cookie!"
i18n.LocalizePlural("Cookies", 4) // "I have 4 cookies!"
i18n.LocalizeTemplateSingle("CookiesX", "X", "chocolate") // "I have chocolate cookies!"
i18n.LocalizeTemplateSingleWithPlural("CookiesX2", 4, "X", "chocolate") // "I have 4 chocolate cookies!"
i18n.LocalizeTemplateX("CookiesABC", "chocolate", "basket", "home") // "I have chocolate cookies in my basket back at home!"
i18n.LocalizeTemplateXPlural("CookiesABC2", 400, "chocolate", "basket", "home") // "I have 400 chocolate cookies in my basket back at home!"

[MJacred]

@nicksnyder, if you could take a look, please.
I can also add build tags, if you'd like