add possibility to disable/uninstall plugins

- add possibility to disable/uninstall plugins.
- move $plugins_host outside of if block, as it looks identical for both cases.
- rename all occurrences of ${name}.hpi with ${plugin}

For uninstall/disable, there are 2 ifs added, each with 2 execs (one for jpi an one for hpi)

  if ($version == 'absent' or $version == 'uninstall') {
   test -f ${plugin_dir}/${plugin}  && rm -rf ${name} ${plugin} ${name}.jpi ${name}.jpi.pinned && touch ${name}.jpi.disabled
   test -f ${plugin_dir}/${name}.jpi -a ! -f ${plugin_dir}/${name}.jpi.disabled && touch ${plugin_dir}/${name}.jpi.disabled
} elsif ($version == 'disable' or $version == 'disabled') {
   test -f ${plugin_dir}/${name}.jpi -a ! -f ${plugin_dir}/${name}.jpi.disabled  && touch ${plugin_dir}/${name}.jpi.disabled
   test -f ${plugin_dir}/${plugin} -a ! -f ${plugin_dir}/${plugin}.disabled         && touch ${plugin_dir}/${plugin}.disabled
} else {
## previous code and added a resource to remove any "disabled" files
}