Replaced a few calls to stringr with stringi (which has to stay for unicode encoding function). Not likely that people won't have stringr, but makes the dependencies a little better.

Fixes issues with adding <span> and </span> around zero-length groups and more generally groups that start and end at the same index.

Ensures that g00 wraps all others, then at index with close/opens, closes open groups first before opening new groups.

- Colorize regex in view_regex
- Use exact=TRUE param to show all backslashes in wrap_regex
- Rename wrap_span -> wrap_result