Log Message:
po4a: update to 0.55.

AsciiDoc:
 * Fix Asciidoc unindented lists (GitHub's #149)
 * Make Asciidoc Tables nowrap and support current format (GitHub's #63)
 * Do not include table fences in pot file (GitHub's #163)
 * Add option to prevent translation of target of image blocks (GitHub's #164)

Markdown:
 * Improve markdown ruler parsing, and add test.
 * Add markdown fenced code block parsing, and add test.

Sgml:
 * Avoid deprecated unescaped left brace in regex to get ready for Perl 5.32.
   (Debian's #903735)

TransTractor:
 * Ensure to split lines before addendum operation without loss or addition of
   newline.  With this change, addendum behavior is more intuitive.
   (Debian's #518218, Github's #147, #153)

Wml:
 * Fix longstanding error regarding comments at the beginning of a file.
   (Github's #138)

Xml, Docbook:
 * Document XML tag behavior customization with example to help use case
   specific customization.  (Debian's #515763)
 * Debug output enhancement to help people understand what exactly happening
   inside po4a.
 * Extensive POD and code comment additions and updates.

po4a tool:
 * Add pot_in feature to support the secondary master file for the base of
   POT/PO file generation.

Test:
 * Add XML test cases with po4a including addendum, tag customization, and
   pot_in feature.

Translations:
 * New translation to Chinese and Chinese Simplified, thanks Liu Guang.
 * Updated: Dutch, thanks Frans Spiesschaert.
 * Updated: Estonian, thanks Kristjan Räts.
 * Updated: French, thanks Brandelune and ButterflyOfFire.
 * Updated: German, thanks Helge Kreutzmann.
 * Updated: Italian, thanks Marco Ciampa.
 * Updated: Norwegian Bokmål, thanks Petter Reinholdtsen.
 * Updated: Portuguese (Brazil), thanks Rafael Fontenelle and Luiz
   Fernando Ranghetti.
 * Updated: Russian: thanks Igor.
 * Updated: Spanish, thanks emma peel and Omar Campagne.
 * Updated: Swedish, thanks Anders Jonsson.
 * Updated: Ukrainian, thanks Yuri Chornoivan (українська).
 * Updated: Vietnamese, thanks Trần Ngọc Quân.