add view panel to feednix

Hi, I made some changes to feednix to fit my needs, and I'm pulling a request, so you can merge or rewrite them if you find them useful
* use $TMPDIR (mkdtemp) for preview.html file to avoid unneeded disk access
* add a new panel to shown a quick preview for current selected item (html formated by w3m) instead of the need to open w3m for previews
* add 3 new config options to add some interface tweaks (ctg_win_width: to change ctgWin width, view_win_height: to set viewWin height, view_win_height_per: to set viewWin height percent that can be used instead of the previous option and overwirte it)
* fix unreading unreaded post when handling "u"
* fix mark as read aleardy marked as read posts on "O" handling

demo of view panel with custom config options (ctg_win_width=15, view_win_height_per=50):

 ![2014-08-04-210123_484x316_scrot](https://cloud.githubusercontent.com/assets/2295241/3801969/f4b4d49e-1c09-11e4-8055-5f8934f348b5.png)