Add options to not include <title> and <desc>.

As detailed in #82 (including use cases), it would be useful to be able to exclude `<title>` and `<desc>` if these are already being set in app code.

## Changes
- Adds two options: `includeTitleElement` and `preserveDescElement`. Both are `true` by default.
- Adds tests
- Updates readme with new options (including some guidance on when it’s OK to turn off `<title>`)
- Updates version number and changelog

## Notes

I’m open to removing `preserveDescElement` from the PR. My main goal is to allow turning off `<title>`s which are otherwise always added. If you are considering this PR, I’m also happy to resubmit with changes, for instance if you prefer different name(s) for the options e.g. `suppressAccessibleTitle`.