Add :buffer Ex command

Mostly compatible with Vim. The incompatibilities are

 - There must be a space between the command and the buffer reference, even if
   that reference is a number. E.g., vim supports `:b4` but this command
   requires that to be `:b 4`.

 - A trailing `!` causes the command to not be recognized.

 - The buffer command behaves as if a trailing `!` had been added.

 - There is no completion support.
Add :buffers command

The output needs improvement. It doesn't include all of the information that
vim provides and the status bar doesn't support including newlines.
Improve output of :buffers command

The output contains the buffer number and has one entry per-line. As the
status bar output currently doesn't support including newlines, a new buffer
is created.
<C-6> and <C-^> normal mode commands
Missing synonyms for :buffer
Merge pull request #485 from benarmston/vim2-buffer-and-buffers-ex-commands

Vim2 improve buffer navigation