i3status: Allow customization of module separator

i3status: Allow customization of module separator

This patch adds the ability to customize the separator that is placed
between modules.

Specifically this patch:

* adds the "separator" general directive
* moves the definition of the default separator for the different
  output formats (excluding color formatting) to src/i3status.c
* updates the SEC_CLOSE_MAP macro to disable the separator for the
  i3bar output format if the separator directive dictates so
* changes print_seperator() in src/output.c to take a separator
  parameter in order to disable the output of the separator if
  the separator is empty and to use the provided separator
  otherwise
* updates the manpage to explain the new directive