Feature to disable add files button

[anmol1707]

Description

Added a prop that decides whether or not to show the add files button in the input bar. Closes #134.

Checklist

• [x] Additions/changes follow the current code structure
• [x] All changes are related to a single topic
• [x] Demo tested (npm start) to ensure it still functions as expected, and/or
• [x] New features are demonstrated to the demo (leave checkbox blank if not applicable)
• [x] Readme/docs are updated
• [x] npm run lint passes (npm run lint -- --fix will autofix where possible)