* Apparently snapshot no longer stores class of message?
* Doesn't appear any of the messages themselves are altered...

* `get_bookmarks_pdftools()` gets pdf bookmark information using `pdftools::pdf_toc()` (#30).
  Note though due to poppler library limitations it can't return page numbers that correspond to each bookmark.
  Hence `get_bookmarks()` will try to use the following helper functions in the following order:

  1. `get_bookmarks_pdftk()`
  2. `get_bookmarks_pdftools()`
* Improved support for reading/writing Unicode values in non-Unicode locales (#57).
* `get_bookmarks()` and `get_bookmarks_pdftk()` no longer returns its data frame value invisibly.

closes #30, progress on #57

* New `get_bookmarks_pdftools()` is not good enough
  for use with `set_bookmarks()` since doesn't know page numbers.
  Need to make sure `get_bookmarks_pdftk()` will be called
  in examples/tests otherwise skip.

* ghostscript v10.3 breaks code to disable old bookmarks
* Now skip tests that verify old bookmarks disabled if detects
  ghostscript version is strictly greater than 9.55

see #59

* We can again disable old bookmarks when setting new bookmarks
  in `set_bookmarks_gs()` for the newest versions of `ghostscript`.
  The new approach means we could remove some GPLv3 code and have relicensed the package under the MIT license.
* Instead of using W. Trevor King's postscript files to disable old
  bookmarks we use the (previously undocumented) `-dNO_PDFMARK_OUTLINES`
  flag of `ghostscript`.

closes #59

* Can't submit package to CRAN with non-working links
  and exiv2.org currently doesn't work so instead
  replace with a link to Github repo.
* See Exiv2/exiv2#2875 for more info
  on exiv2.org is currently down.

[skip ci]