man.vim: convert ftplugin to actual plugin.

- do not create leader maps
- :norm! instead of :norm
- :keepjumps during layout
- use blackhole reg to avoid polluting unnamed reg
- format buffer name as "man://foo(2)"
- simulate behavior of `man`
    - buffer-local mapping of q to quit
    - open in new tab instead of new window
    - set 'nolist'
    - set tabstop=8

runtime/plugin/man.vim

@@ -0,0 +1,6 @@
+if get(g:, 'loaded_man', 0)
+  finish
+endif
+let g:loaded_man = 1
+
+command! -nargs=+ Man call man#get_page(<f-args>)