It is often useful to be reminded that you are, in fact, in the middle
of a debug or interactive install. We provided this reminder in the form
of HOMEBREW_DEBUG_INSTALL, but we can make this even easier for the end
user to consume by exposing it in the form of a shell function.

When HOMEBREW_DEBUG_INSTALL is set, the __brew_ps1() function returns
the string "(formula_name|DEBUG)" by default (much like the __git_ps1()
output when performing some long-running operation, e.g.
"(branch|REBASE-i)". The formatting around "formula_name|DEBUG" can be
customized by passing a format string to the function.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>