Release Newsboat 2.13

Lists below only mention user-visible changes, but I would also like to
acknowledge contributions from the following people: Anatoly Kalin, Eduardo
Sanchez, Friedrich von Never, Kamil Wsół, glacambre.

Added
=====
- Respect TMPDIR environment variable when writing temporary files (ಚಿರಾಗ್ ನಟರಾಜ್)
    (#250)
- `delete-all-articles` operation that marks all articles in the feed as deleted
    (Kamil Wsół)

Changed
=======
- Require `cookie-cache` setting if Newsblur API is used (Alexander Batischev)
- Translations: Russian, Ukraininan (Alexander Batischev), Swedish (Dennis
    Öberg), German (Lysander Trischler)
- json.hpp updated to version 3.2.0
- Natural sort order for article titles, so numbers are put in the expected
    order (e.g. 1, 2, 5, 10, 11 rather than 1, 10, 11, 2, 5) (Nikos Tsipinakis)

Fixed
=====
- Do not create empty files if history is disabled (Nikos Tsipinakis)
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