[Link] Use xdg-open

Use the default application to open link with the eponymous plugin.

It's OS dependent so:

• Linux: use "xdg-open"
• Windows: use "start"
• MacOS: use "open"

Attached is a proposed implementation patch.

• Changesets:
  • Revision ba32d74a by MikaYuoadas on 2014-06-26T13:56:32Z:

Use xdg-open by default in the /link plugin

fix #2550

• Uploads:
  • poezio-default-open.patch