Add small gettext plugin

Author: TheAssassin

gettext/README.md

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+# linuxdeploy-plugin-gettext
+
+This plugin installs an AppRun hook to make sure applications which use [gettext](https://en.wikipedia.org/wiki/Gettext) load localization data from within the AppImage.

gettext/linuxdeploy-plugin-gettext.sh

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+#! /bin/bash
+
+# exit whenever a command called in this script fails
+set -e
+
+appdir=""
+
+show_usage() {
+    echo "Usage: bash $0 --appdir <AppDir>"
+}
+
+while [ "$1" != "" ]; do
+    case "$1" in
+        --plugin-api-version)
+            echo "0"
+            exit 0
+            ;;
+        --appdir)
+            appdir="$2"
+            shift
+            shift
+            ;;
+        *)
+            echo "Invalid argument: $1"
+            echo
+            show_usage
+            exit 2
+    esac
+done
+
+if [[ "$appdir" == "" ]]; then
+    show_usage
+    exit 2
+fi
+
+echo "Installing AppRun hook"
+set -x
+mkdir -p "$appdir"/apprun-hooks
+cat > "$appdir"/apprun-hooks/linuxdeploy-plugin-gettext.sh <<\EOF
+export TEXTDOMAINDIR="$this_dir"/usr/share/locale:"$TEXTDOMAINDIR"
+EOF