restored behavior of respecting config values unless set by command flags. fixed #116
Truncated; .Site.Params; First function

 * Add `.Truncated` bool to each page; will be set true if the
   `.Summary` is truncated and it's worth showing a "more" link of some
   kind.
 * Add `Params` to the site config, defining `.Site.Params` accessible
   to each page; this lets the site maintainer associate arbitrary data
   with names, on a site-wide basis.
 * Provide a `First` function to templates:
   * Use-case: `{{range First 5 .Site.Recent}}` or anything else which
     is a simple iterable provided by hugolib
 * Tests by me for `.Truncated` and `First`

Also @noahcampbell contributed towards this:

 * Add UnitTest for `.Site.Params`:
   > Digging into this test case a bit more, I'm realizing that we need
   > to create a param test case to ensure that for each type we render
   > (page, index, homepage, rss, etc.) that the proper fields are
   > represented.  This will help us refactor without fear in the
   > future.