No template keyword whether a component list has a certain value

[jwillemsen]

For solving #50 I want to filter out any element from template_files from header_files. It would be helpful to have a way to check if template_files has a certain element like below, can't find something in MPC that does this

<%foreach(uniq(header_files))%>
<%!exists(template_files, header_file)%>
do something
<%endexists%>
%<endfor%>