Append, rather than prepend to PROMPT_COMMAND
Replace an echo with a redirect
Handle empty PROMPT_COMMANDS
remove mktemp dependency

Closes #103
Merge branch 'master' into prompt_command_append

Conflicts:
	z.sh
Indentation is 4 spaces.

When I was a feckless youth, I preferred single space indentation for
shell. Now in my wise old age that feels cramped. Don't even say
anything about tabs - I will slap you.
Merge rupa/master into prompt_command_append
Alternate approach for appending to PROMPT_COMMAND

Our basic problem with appending to the end of $PROMPT_COMMAND was that
we couldn't put 2 semicolons in a row.  We can, however, put 2 newlines
in a row, which seems to be treated identically.  As far as I can tell,
this approach works whether PROMPT_COMMAND is unset, set to an empty
string, or set to a non-empty string with or without a trailing
semicolon.