adds support for getitems in argstring formatting

[tclose]

Types of changes

• New feature (non-breaking change which adds functionality)

Summary

Adds support for using getitem within argstr formatters, e.g.

"-coord {coord[0]} {coord[1]}..."

Checklist

• [x] I have added tests to cover my changes (if necessary)
• [x] I have updated documentation (if necessary)