Release Newsboat 2.18

Lists below only mention user-visible changes, but I would also like to
acknowledge contributions from the following people: Simon Schuster, and seanBE.

Added
=====
- Logo by noobilanderi

Changed
=======
- `open-in-browser-and-mark-read` no longer marks item read if browser returned
    a non-zero exit code. Similarly, `open-all-unread-in-browser` and
    `open-all-unread-in-browser-and-mark-read` abort on non-zero exit code
    (Marco Sirabella)
- Update vendored version of Catch3 to 2.11.0
- Update vendored version of nlohmann/json to 3.7.3
- Update translations: German (Lysander Trischler), Russian, Ukrainian
    (Alexander Batischev)

Fixed
=====
- `unbind-key -a` breaking cmdline, search and goto-url (#454) (kmws)
- Flaky `run_command()` test (Alexander Batischev)

Security
========
- smallvec crate bumped to 0.6.10, to get fixes for RUSTSEC-2019-0009 and
    RUSTSEC-2019-0012
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