Add interactive buffer pick mode

[Crote]

This adds a command which enables interactive buffer selection, by dynamically creating a user mode from the buffer info.

The reason for adding this is that I sometimes do :info-buffers, but when I enter the command to switch to the buffer I want, I'll have forgotten which buffer I actually wanted to switch to. In the buffer pick mode, one can directly switch to a buffer while the information is still displayed on the screen.

Some notes:

• The keys are a bit subjective. This seemed like a logical order to me. The initial keys are also the same as those used for the buffer info, so that is probably easy to remember.
• There is unfortunately no clean way to show that the key limit has been reached. The limit could of course be raised by adding uppercase and other keys, but I don't think it is common to reach this limit anyway. Perhaps a dummy mapping could be added to display that the limit has been reached, or some form of pagination could be added.
• It might be nice to integrate this with the existing buffer mode, but the obvious keys have already been taken.
• The display style is a bit different than the info box. It might be nice to unify this a little.

[caksoylar]

I ran into issues trying to use this function today. Looks like my sh does not like the ${keys:$index:1} syntax, this is what I get in the debug buffer after calling pick-buffers:

shell stderr: <<<
sh: 16: Bad substitution
>>>

Indeed when I try doing something similar in sh, I get the same error:

$ sh -c 'keys=abcd; index=0; echo ${keys:$index:1}'
sh: 1: Bad substitution

It works fine with bash. My sh maps to /bin/dash in Ubuntu 18.04.