Simplify API with only top-level functions

[mgeisler]

This is a breaking change which simplifies and stream-lines the API in several ways:

• Instead of having both Wrapper::wrap and a top-level textwrap::wrap function, we now only have the textwrap::wrap
  function. This should simplify things a little since there is now just one way to use the API.

• The Wrapper struct has been turned into an Options struct.

• The API of textwrap::wrap and textwrap::fill functions have changed in a backwards compatible fashion. They still take a string and a width:

  textwrap::fill("some string", 10);
  

• In addition to a usize width, you can now also pass Options as the second argument:

  textwrap::fill("some string", Options::new(10).break_words(false));
  

Migration from using Wrapper is simple: change a call to Wrapper::fill to fill:

use textwrap::Wrapper;
let wrapper = Wrapper::new(10).initial_indent("  ");
wrapper.fill("some string")

into

use textwrap::{Options, fill};
let options = Options::new(10).initial_indent("  ");
fill("some string", &options)