typeass: mark "s/pattern/replacement/" with ticks
urlselect: add custom external command support

- You can add custom 10 external commands using
  `plugins.var.lua.urlselect.ext_cmd_?` where ? is number from 0 to 9.
  While selecting you can press the number to launch the command on
  selected url.

- Switch to weechat.hook_process() to make weechat happy