[Feature] Allow input of additional information by signees

[ronald-jaepel]

Is your feature request related to a problem? Please describe.
Our workflow requires us to obtain the legal name and, whenever applicable, the employer info of our signees.

Describe the solution you'd like
A straightforward way to ask the signees for that information without having to host a server/website. Both options mentioned in #85 in this comment would work for us but we can't find how to implement them.

Describe alternatives you've considered
Using the full-featured cla-assistant.

Thank you for the great work on the cla-assistant!