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-script.5.scd

@@ -133,10 +133,9 @@ bar:
             title|string|{{title}}
         content:
           map:
-            tag: status
-            values:
-              Paused: {empty: {}}
-              Playing:
+            conditions:
+              (status == Paused): {empty: {}}
+              (status == Playing):
                 content: {string: {text: "{artist} - {title}"}}
 ```