2.11: Improved highlighting

This release properly highlights the following kinds of changes:

- Some lines changed, plus extra lines added at the end
- Some lines changed, plus extra lines added at the beginning
- Some lines changed, plus some lines removed at the end
- Some lines changed, plus some lines removed at the beginning

For example, this means that if you have a multi line list in your source code,
and add a comma to one line, plus a number of more lines after that, the comma
will now be highlighted.

Highlighting the rest of those lines doesn't provide value since they are all
new, which should be obvious anyway.

2.10: Highlight common parts of shortened sections

Test case here:
https://github.com/walles/riff/blob/master/testdata/shorten-section.diff

2.9: Support M1 / Silicon macs

By making universal binaries (for both x86 and AMD64 on mac), PR #18, thanks @xeago!

Also, fix a broken tip in --help, PR #19, thanks @xeago!

2.8: Improve external diff error handling

* Treat exit code 1 (differences found) as success
* Verify file / dir readability before calling external diff

2.7: Do partial refines

Let's say somebody does:

- FOO
+ FOO,
+ BAR,
+ BAZ

Before this release, we didn't highlight this, because too many lines changed.

Starting with this release, we now highlight the "," on the "+ FOO," line,
because that's a change of an existing line.

2.6: Fix crash if pager is closed early.

Fixes:
* <https://github.com/walles/riff/issues/10>
* <https://github.com/walles/riff/issues/16>

2.5: Count _ as part of words when tokenizing

This was issue #15, the change turns one example from hard to read to easy to read.

2.4

Performance improvements

2.3: Highlight spaces between highlighted words

This is our way of saying "this part of the sentence was replaced", rather than
"these individual words were replaced".

2.2: Support diffing paths: "riff a.txt b.txt"

Works on directories or files.