Add 'options' argument to astrometry_net()
Update docstring of astrometry_net()
Add additional_options_callback() module-level function
Add -o option
Add Bash completion for -o
Update call to astrometry_net()
Add -o option

As we saw in issue #36, our users may need to run Astrometry.net's
solve-field with options other than those supported by 'astrometry'.
This option allows us to specify additional options that are passed to
solve-field, instead of forcing us to edit the source code in order to
modify the argument list. These options and the arguments that they
take, if any, must be given as a single string — for example: -o
"--downsample = 2", -o "--scale-low 0.55" or -o "--invert".

This merges branch 'add-o-option'