Need to do better meta data matching. right now only type is used, future a tool should have
<tool>
<input name="xxx" ...>
<required name="
http://purl..../format">image/jpeg</required>
<optional name="
http://purl..../title">FOO BAR</optional>
</input>
</tool>
This can also be used to do platform specific tools, ie a required option can be os.name == windows