Add a CommandLineConverter converter

Converter allowing to execute a command line.

The goal is to be able to contribute a command line and a converter using it, without creating a Java class for the converter.

Sample contribution:

<converter name="myConverter"
  class="org.nuxeo.convert.CommandLineConverter">
  <parameters>
    <parameter name="CommandLineName">myCommandLine</parameter>
    <parameter name="">...</parameter>
  </parameters>
</converter>

See https://github.com/nuxeo/nuxeo-labs/blob/master/nuxeo-labs-images/src/main/java/org/nuxeo/labs/images/GenericImageMagickConverter.java and https://github.com/nuxeo/nuxeo-labs/blob/master/nuxeo-labs-images/src/main/resources/OSGI-INF/extensions/conversions.xml

See if we can align (or even deprecate) PDF2HtmlConverter, PDF2ImageConverter and WordPerfect2TextConverter on this new converter.

Make sure it is possible to send parameters from the converter to the command line (same names?).