Make readers work with new move_spectral_axis keyword

[rosteen]

#1033 allows the spectral axis to be anywhere, rather than forcing it to be last, in specutils 2.0. This also added a move_spectral_axis keyword to Spectrum1D.__init__ to allow users to replicate the old behavior, or force the spectral axis to be in another location. However, that PR did not give the option to input that keyword to Spectrum1D.read and have it propagate through to the object initialization - this issue is to do that follow-up work.