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> ""

test/full-conf-good.yml

@@ -62,10 +62,9 @@ bar:
     - clock:
         content:
           map:
-            tag: date
             default: {string: {text: default value}}
-            values:
-              1234: {string: {text: specific value}}
+            conditions:
+              date == 1234: {string: {text: specific value}}
     - clock:
         content:
           progress-bar: