Moment.js-style escaping of literal text in date/time formats

[stefanholzmueller]

See https://github.com/moment/momentjs.com/blob/master/docs/moment/04-displaying/01-format.md#escaping-characters

P.S.: If anyone knows a neater way to construct a parser for this, I'd be happy to learn about it.

[stefanholzmueller]

By reading #22 I just realized that one can avoid the string parsing alltogether and use FormatterCommands directly and include a Placeholder with the desired literal text. This is arguably safer and more flexible.

Also, adding escaping would be a breaking change. I don't think this is really needed.