Add template options.

As decided in #93.

This adds an optional "options" field to a template's section in config file.

Then we can do:
<pre>
options {
     key = value
}
</pre>
After that, when the template is rendered, we can call ${Options.key} to render it and also do ${if Options.key} which will return false if the key is null or evaluates to "false" and true otherwise.