awk: Recognize .twjr file extension
* gettext-tools/src/x-awk.h (EXTENSIONS_AWK): Register file extensions
".gawk" and ".twjr". Suggested by Karl Berry in:
<https://lists.gnu.org/archive/html/bug-gettext/2015-05/msg00032.html>.
* gettext-tools/doc/gettext.texi (gawk): Mention new file extensions
".gawk" and ".twjr".
