see if a command is available or not. If more
than one command is present (for instance "ls -a | grep foo") then this
just checks the first.

Note that shell (like cd and ulimit) aren't in the PATH so this lookup will
try to assume that it's available. This only happends for recognized shell
commands (those in SHELL_COMMANDS).

:param str command: command to search for
:param bool cached: makes use of available cached results if **True**

:returns: **True** if an executable we can use by that name exists in the
  PATH, **False** otherwise
Ú