Add option to perform unicode normalisation

[adamobeng]

Signed-off-by: Adam Obeng github@binaryeagle.com

[kbenoit]

Needs work on a few fronts:

• [ ] the encoding tests are failing because of issue #55
• [ ] no @param entries for the new readtext() arguments replace_special_characters, normalize_unicode
• [ ] could use some tests and/or documentation for the internal functions
  • make_character_class_replacements()
  • unicodeNorm()
    I suggest we use standard roxygen2 documentation but not export these, and mark them as @keywords internal. And add tests.