Add option for end user to specify payment amount

Problem/Motivation

I'm implementing a donation button. This module would be ideal for the purpose if it was possible for the end user to specify an amount rather than having a hard coded price.

Steps to reproduce

n/a

Proposed resolution

On the field widget add a checkbox for "user can specify amount".

If ticked, an amount field would appear above the Pay button,

Remaining tasks

User interface changes

API changes

Data model changes