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> ""
When set to a non-negative value, the script module will call the
configured script every <poll-interval> second.
In this mode, the script is expected to write one tag set and then
exit.
This is intended to simplify the implementation of scripts that would
otherwise just do a loop + sleep.
Closes#67
