Introduce a new configuration variable, BASH_EXEC_IGNORE_LIBRARIES. If
set to true, the behaviour of bash-exec is modified: if a shell file has
a file extension and no shebang line, it is ignored, i.e., allowed to be
non-executable. This allows files that are only every sourced from other
shell files, acting as libraries and not executables, to have no
executable bit set without failing the bash-exec linter.
