Add a trace mode.

If the -x option is used, or the PLUGINHOOK_TRACE env var is non empty, pluginhook will log calls to hooks on stderr.

Example output : 

    $ echo Hello | PLUGIN_PATH=./tests/plugins ./pluginhook -x pre-build ABC DEF
    Executing :  [tests/plugins/bar/pre-build ABC DEF]
    Executing :  [tests/plugins/foo/pre-build ABC DEF]
    Hello
    bar pre-build DEF
    foo pre-build ABC DEF