feat: support option `preserveSignificantWhitespace`

[kendrickw]

Fixes #478

Add new option preserveSignificantWhitespace, to preserve significant whitespace within XML elements. This PR should address all the cases where significant white space exists.

Specifically:

• spaces within string are preserved:

  // before and after prettier identical
  <v>some string    extra space</v>
  

• string with space(s) at the beginning and/or end:

  // before and after prettier identical
  <v>
    some  string
  </v>
  

• mixing elements within string literal is also supported, space(s) surrounding and within string literal will be preserved:

  before: <v> some  string<abc/>more string </v>
   after: <v> some  string<abc />more string </v>