teach planemo to help users with different osx/unix sed syntax

Possibly even other 'usual' commands. Could this be a part of the linting tool?

Details:
http://unix.stackexchange.com/questions/13711/differences-between-sed-on-mac-osx-and-other-standard-sed

Example of package failing on OSX:
https://github.com/galaxyproject/tools-iuc/blob/master/packages/package_perl_5_18/tool_dependencies.xml

because of this command:

`sed -i 's|#!$INSTALL_DIR/bin/|#!/usr/bin/env |' $INSTALL_DIR/bin/*`