add N marker for displaying full track info and make n show only a number

generated filename was ugly if track tag was in format
xx/xx, so %n should display only current track number.

doc/ncmpcpp.1

@@ -295,7 +295,8 @@ For song format you can use:
  %t - title
  %b - album
  %y - year
- %n - track number
+ %n - track number (01/12 -> 01)
+ %N - full track info (01/12 -> 01/12)
  %g - genre
  %c - composer
  %p - performer