Encourage use of lines with newline characters

[trishume]

I'd like to change the HTML APIs and the doc examples to use the newlines version of the syntax sets. The nonewlines version is a hack and sometimes provides incorrect results.

Here's my proposed plan:

• @robinst can you contribute your iterator from your Stack Overflow post to syntect under the easy or util module with a doc comment explaining how it lets you use the newlines version with strings? Stack Overflow's default license isn't compatible with me copy-pasting it AFAIK.
• Use that iterator in the html::highlighted_snippet_for_string and the IO with newline reader loop in highlighted_snippet_for_file.
• Rename both of those methods, not because they particularly need renaming but so that people upgrading don't miss the breaking change and get subtly broken behaviour.
• Change all the examples to use the newlines version of the syntax sets as required

[robinst]

Good idea! Raised a PR for the first point.

We should also change the docs for load_defaults_nonewlines and load_defaults_newlines so the documentation for load_defaults_newlines is the main one, and the other one refers to it. At the moment, the docs make it sound like the "nonewlines" is the main one:

/// Same as `load_defaults_nonewlines` but for parsing line strings with newlines at the end.