resourceloader: Rewrite the minify.php maintenance script

resourceloader: Rewrite the minify.php maintenance script

* Remove the options of writing to a directory or a ".min.EXT" file,
  this made sense long ago when the script was used for producing
  final artefacts to check into SVN. But not today when it is mainly
  for testing out minifier behaviour.

* Add support for reading from standard input.
* Add support for setting --type (especialy for standard input).
* Add support for writing to standard output.

* Make --outfile optional. Write to standard output by default.

* Register <file> via Maintenance::addArg(), so that it shows
  up in --help. And so that we don't need our own validation for
  this as a required argument.

Test Plan:

  Run `echo 'x = true;' | php maintenance/minify.php --type js -`
  Output should be "x=!0;"

Change-Id: I0fdbedc713f66b9bd25eb8cb7334252cef7e20a8