Implement conditions on tag

A condition is formed by:
    <tag> <op> <value>

<tag> is the normal yambar tag. <op> is one of '==', '!=', '<', '<=', '>', or
'>='. <value> is what you wish to compare it to.

'boolean' tags must be used directly. They falsehood is matched with '~':

    <tag>
    ~<tag>

Finally, to match an empty string, one must use ' "" ':
    <tag> <op> ""

doc/yambar-modules-river.5.scd

@@ -79,10 +79,9 @@ bar:
         title: {string: { text: "{seat} - {title}" }}
         content:
           map:
-            tag: occupied
-            values:
-              false: {empty: {}}
-              true:
+            conditions:
+              (occupied == false): {empty: {}}
+              (occupied == true):
                 string:
                   margin: 5
                   text: "{id}: {state}"