Added an "sdescription" property to return only the entry description

The current behavior of the `description` property automatically extracts the first 50 characters of the source, which may contain Markdown code instead of text. This also prevents the caller from distinguishing whether a description was explicitly set in the front matter.

Instead, `sdescription` returns an empty string if a description was not set, and allows the summary to be performed inside the template like so:

     <meta name="description" content="{{ content.sdescription or content.entry|striptags|summarize }}">

which guarantees proper text since `content.entry` is already processed HTML.

For lack of a better name, `sdescription` was chosen for "string description" because there was already `imonth`, `iday`, etc which I believe represents "integer *something*".