Time Lord technology: for_window config directive to run arbitrary cmds

An example to set all XTerms floating:
    for_window [class="XTerm"] mode floating

To make all urxvts use a 1-pixel border:
    for_window [class="urxvt"] border 1pixel

A less useful, but rather funny example:
    for_window [title="x200: ~/work"] mode floating

The commands are not completely arbitrary. The commands above were tested,
others may need some fixing. Internally, windows are compared against your
criteria (class, title, …) when they are initially managed and whenever one of
the relevant values change. Then, the specified command is run *once* (per
window). It gets prefixed with a criteria to make it match only the specific
window that triggered it. So, if you configure "mode floating", i3 runs
something like '[id="8393923"] mode floating'.

src/main.c

@@ -37,6 +37,8 @@ struct assignments_head assignments = TAILQ_HEAD_INITIALIZER(assignments);
  * output) */
 struct ws_assignments_head ws_assignments = TAILQ_HEAD_INITIALIZER(ws_assignments);
 
+struct real_assignments_head real_assignments = TAILQ_HEAD_INITIALIZER(real_assignments);
+
 /* We hope that those are supported and set them to true */
 bool xcursor_supported = true;
 bool xkb_supported = true;