Description of proposed changes

Add view.py --faultline --faultline-linewidth options to plot faults using GMT lonlat file.

• utils.arg_utils.add_map_argument(): add --faultline and --faultline-lw options

• utils.plot: add plot_faultline()

• view.plot_slice: call pp.plot_faultline()

• cli.view: add example usage

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.