i3 config: Add mod+m & mod+' to mark & goto windows

Works like vim's mark and goto feature.

- gen_config.sh is expanded to allow config fragments to be generated by
  scripts instead of static fragments. If the script doesn't find one of
  the fragment filenames it is given it now checks to see if there is a
  script of the same name with a .sh or .py extension (adding more
  extensions is trivial).

  This means, the name of the script should be specified WITHOUT the
  extension, otherwise it would be included verbatim.

- The marks.sh script generates "mark" and "goto" modes and fills in all
  the key bindings.

- The default for gen_config.sh now includes the marks script.

- I've changed the split horizonal & vertical bindings to both be on the
  semicolon since I want to use the same character for goto as vim. So
  far I've only used these by accident.