Add sorting method 'case-insensitive'

[bartholomej]

There is a new option for sorting.

What is in this PR?

• New parameter (--sort case-insensitive) is available
• Updated readme
• Updated tests
• No breaking changes because default option --sort (without parameter) is the same as it was

Why?

It can be useful when you are using one of the online i18n services (e.g. Phrase, Crowdin, ...), which export alphabetically sorted translations (but in case insensitive mode).
The output from ngx-translate-extract will therefore be compatible, easier to merge and git diff will be much clearer.

Examples

--sort (default js .sort() is case sensitive)

{
  "9": "9",
  "88": "88",
  "Bat": "Bat",
  "ZZ-top": "ZZ-top",
  "ast": "ast"
}

--sort case-insensitive (new method)

{
  "9": "9",
  "88": "88",
  "ast": "ast",
  "Bat": "Bat",
  "ZZ-top": "ZZ-top"
}