Support different encodings

[aslakhellesoy]

Cucumber-Ruby and Cucumber-JVM currently supports this:

# encoding: iso-8859-1

We should build support for this into Gherkin3 too I think. If a File object is passed in - read it first as UTF-8. Look for the header, and if it's present and not UTF-8, read the file again with the specified encoding.

The current implementations are here:

https://github.com/cucumber/gherkin/blob/master/lib/gherkin/lexer/encoding.rb
https://github.com/cucumber/gherkin/blob/master/java/src/main/java/gherkin/lexer/Encoding.java