GitHub uses Linuguist for syntax highlighting [1], and Linuguist
defines two related grammars:

* Shell (alias 'sh') [2], which applies to POSIX and similar shell
  languages.

* ShellSession (alias 'console') [3], which extends the Shell grammar
  with support for prompts and command output as you would usually see
  in an interactive shell session.

This commit consistently uses 'sh' to highlight blocks which contain
only shell commands (without prompts or output).  And it uses
'console' to highlight blocks which contain prompts, commands, and
output.

I've also removed prompts blocks which contain no output, because in
the no-output cases that approach seemed more popular within this
repository.

I've also standardised on the 'sh' alias, where this repository
previously used both 'sh' and 'shell'.

I've also added continuation prompts (PS2) to the long
describe-auto-scaling-groups example, to keep the continued command
from being marked up as output.  And I've replaced the terminal \
continuation there with a terminal |; we need a pipe anyway so we
might as well save a character ;).

[1]: https://help.github.com/articles/creating-and-highlighting-code-blocks/#syntax-highlighting
[2]: https://github.com/github/linguist/blob/v6.2.0/lib/linguist/languages.yml#L4317-L4378
[3]: https://github.com/github/linguist/blob/v6.2.0/lib/linguist/languages.yml#L4379-L4390