Shortcut for MarkupString parser

[malcommac]

Add a new func to String instances which allows you to parse a tagged (html-like) string and return the NSMutableAttributed string.

It will be called renderTags(withStyles:).

Follows an example:

let sourceTaggedString = "<center>The quick brown fox</center>\njumps over the lazy dog is an <italic>English-language</italic> pangram—a phrase that contains <italic>all</italic> of the letters of the alphabet. It is <extreme><underline>commonly</underline></extreme> used for touch-typing practice."
let _ = sourceTaggedString.renderTags(withStyles: [tag_center,tag_italic,tag_extreme,tag_underline])