pager: Add RTV_PAGER to support a reddit-specific setting

Reddit submissions and comments are styled with Markdown.  If you have a
Markdown-aware pager/viewer on your system it is convenient to be able to
use it rather than the default 'less'/'more'/etc. one used for everything
else on the system.  Support a high-priority environment variable that
will be consulted prior to checking $PAGER and/or defaulting to 'less'.

Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>

rtv/__main__.py

@@ -127,6 +127,7 @@ def main():
             ('$RTV_URLVIEWER', os.getenv('RTV_URLVIEWER')),
             ('$RTV_BROWSER', RTV_BROWSER),
             ('$BROWSER', BROWSER),
+            ('$RTV_PAGER', os.getenv('RTV_PAGER')),
             ('$PAGER', os.getenv('PAGER')),
             ('$VISUAL', os.getenv('VISUAL')),
             ('$EDITOR', os.getenv('EDITOR'))]