Display the command that installs wkhtmltopdf (#255)

It is difficult to tell from the output of apt-get which packages were
requested, because they are interleaved with their dependencies.

Also factor it out into a separate script.

script/install.bash

@@ -34,7 +34,7 @@ bash script/install/postgresql.bash || exit 1 # create user & db if it doesn't e
 
 bash script/install/imagemagick.bash || exit 1 # install imagemagick
 
-[ -n "$CI" ] || sudo apt-get install wkhtmltopdf xvfb || exit 1 # for pdfkit; skip in CI
+[ -n "$CI" ] || bash script/install/wkhtmltopdf.bash || exit 1 # for pdfkit; skip in CI
 
 bash script/install/nztrain.bash || exit 1 # fix files & directory structure
 

script/install/wkhtmltopdf.bash

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+cmd="sudo apt-get install wkhtmltopdf xvfb"
+echo "$ $cmd"
+$cmd || exit 1